SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`u754910178_laravel`.`designs`, CONSTRAINT `post_fk_9058591` FOREIGN KEY (`post_id`) REFERENCE

I would like to save design in the database but I got this error on my server here is my livewire design component :-

<?php

namespace App\Http\Livewire\Design;

use App\Models\Design;
use App\Models\Post;
use App\Models\Project;
use App\Models\Team;
use Livewire\Component;
use Spatie\MediaLibrary\MediaCollections\Models\Media;
use App\Listeners\Design\DesignCreatedEmail;
use App\Events\Design\DesignCreated;


class Create extends Component
{
    public Design $design;

    public $posts;
    public $projects;
    public $post;
    public $post_id;
    public $project;

    public array $mediaToRemove = [];

    public array $listsForFields = [];

    public array $mediaCollections = [];

    public $selectedProjectId = null;

    public function addMedia($media): void
    {
        $this->mediaCollections[$media['collection_name']][] = $media;
    }

    public function removeMedia($media): void
    {
        $collection = collect($this->mediaCollections[$media['collection_name']]);

        $this->mediaCollections[$media['collection_name']] = $collection->reject(fn ($item) => $item['uuid'] === $media['uuid'])->toArray();

        $this->mediaToRemove[] = $media['uuid'];
    }

    protected function syncMedia(): void
    {
        collect($this->mediaCollections)->flatten(1)
            ->each(fn ($item) => Media::where('uuid', $item['uuid'])
                ->update(['model_id' => $this->design->id]));

        Media::whereIn('uuid', $this->mediaToRemove)->delete();
    }

    public function mount(Design $design)
    {
        $this->design = $design;
        $this->projects = Project::all(); // Load all projects
        $this->posts = collect();
        $this->initListsForFields();
    }

    public function updatedSelectedProjectId($projectId)
    {
        $this->posts = Post::where('project_id', $projectId)->get();
    }



    public function render()
    {
        return view('livewire.design.create');
    }


    public function submit()
{
    $this->validate();

    // Assign the selected project and post IDs to the design instance
    $this->design->project_id = $this->selectedProjectId;
    $this->design->post_id = $this->selectedProjectId;
    $this->design->save();
    $this->syncMedia();
    event(new DesignCreated($this->design));

    return redirect()->route('admin.designs.index');
}




    protected function rules(): array
    {
        return [
            'design.project_id' => [
                'integer',
                'exists:projects,id',
                'nullable',
            ],
            'design.post_id' => [
                'integer',
                'exists:posts,id',
                'nullable',
            ],
            'mediaCollections.design_design' => [
                'array',
                'required',
            ],
            'mediaCollections.design_design.*.id' => [
                'integer',
                'exists:media,id',
            ],
            'design.statues' => [
                'nullable',
                'in:' . implode(',', array_keys($this->listsForFields['statues'])),
            ],
            'design.confirmation' => [
                'nullable',
                'in:' . implode(',', array_keys($this->listsForFields['confirmation'])),
            ],
            'design.note' => [
                'string',
                'nullable',
            ],
            'design.review' => [
                'nullable',
                'in:' . implode(',', array_keys($this->listsForFields['review'])),
            ],
            'design.team_id' => [
                'integer',
                'exists:teams,id',
                'required',
            ],
        ];
    }

    protected function initListsForFields(): void
    {
        $this->listsForFields['project']      = Project::pluck('name', 'id')->toArray();
        $this->listsForFields['post']         = Post::pluck('title', 'id')->toArray();
        $this->listsForFields['statues']      = $this->design::STATUES_RADIO;
        $this->listsForFields['confirmation'] = $this->design::CONFIRMATION_RADIO;
        $this->listsForFields['review']       = $this->design::REVIEW_RADIO;
        $this->listsForFields['team']         = Team::pluck('name', 'id')->toArray();
    }
}

Here is my design Model

<?php

namespace App\Models;

use App\Support\HasAdvancedFilter;
use App\Traits\Auditable;
use App\Traits\Tenantable;
use Carbon\Carbon;
use DateTimeInterface;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\SoftDeletes;
use Spatie\MediaLibrary\HasMedia;
use Spatie\MediaLibrary\InteractsWithMedia;
use Spatie\MediaLibrary\MediaCollections\Models\Media;

class Design extends Model implements HasMedia
{
    use HasFactory, HasAdvancedFilter, SoftDeletes, Tenantable, InteractsWithMedia, Auditable;

    public $table = 'designs';

    protected $appends = [
        'design',
    ];

    protected $dates = [
        'created_at',
        'updated_at',
        'deleted_at',
    ];

    public const CONFIRMATION_RADIO = [
        'confirmed' => 'Confirmed',
        'try again' => 'Try again',
        'edit'      => 'Edit',
    ];

    public const STATUES_RADIO = [
        'in progress' => 'In Progress',
        'published'   => 'Published',
        'scheduled'   => 'Scheduled',
    ];

    protected $fillable = [
        'project_id',
        'post_id',
        'statues',
        'confirmation',
        'note',
        'review',
        'team_id',
    ];

    public const REVIEW_RADIO = [
        'not bad'   => 'Not Good',
        'good'      => 'Good',
        'very good' => 'Very Good',
        'excellent' => 'Excellent',
        'brillent'  => 'Brillent',
    ];

    public $orderable = [
        'id',
        'project.name',
        'post.title',
        'created_at',
        'statues',
        'confirmation',
        'note',
        'review',
        'updated_at',
        'deleted_at',
        'team.name',
    ];

    public $filterable = [
        'id',
        'project.name',
        'post.title',
        'created_at',
        'statues',
        'confirmation',
        'note',
        'review',
        'updated_at',
        'deleted_at',
        'team.name',
    ];

    protected function serializeDate(DateTimeInterface $date)
    {
        return $date->format('Y-m-d H:i:s');
    }

    public function registerMediaConversions(Media $media = null): void
    {
        $thumbnailWidth  = 50;
        $thumbnailHeight = 50;

        $thumbnailPreviewWidth  = 120;
        $thumbnailPreviewHeight = 120;

        $this->addMediaConversion('thumbnail')
            ->width($thumbnailWidth)
            ->height($thumbnailHeight)
            ->fit('crop', $thumbnailWidth, $thumbnailHeight);
        $this->addMediaConversion('preview_thumbnail')
            ->width($thumbnailPreviewWidth)
            ->height($thumbnailPreviewHeight)
            ->fit('crop', $thumbnailPreviewWidth, $thumbnailPreviewHeight);
    }

    public function project()
    {
        return $this->belongsTo(Project::class);
    }

    public function post()
    {
        return $this->belongsTo(Post::class);
    }

    public function getDesignAttribute()
    {
        return $this->getMedia('design_design')->map(function ($item) {
            $media                      = $item->toArray();
            $media['url']               = $item->getUrl();
            $media['thumbnail']         = $item->getUrl('thumbnail');
            $media['preview_thumbnail'] = $item->getUrl('preview_thumbnail');

            return $media;
        });
    }

    public function getCreatedAtAttribute($value)
    {
        return $value ? Carbon::createFromFormat('Y-m-d H:i:s', $value)->format(config('project.datetime_format')) : null;
    }

    public function getStatuesLabelAttribute($value)
    {
        return static::STATUES_RADIO[$this->statues] ?? null;
    }

    public function getConfirmationLabelAttribute($value)
    {
        return static::CONFIRMATION_RADIO[$this->confirmation] ?? null;
    }

    public function getReviewLabelAttribute($value)
    {
        return static::REVIEW_RADIO[$this->review] ?? null;
    }

    public function getUpdatedAtAttribute($value)
    {
        return $value ? Carbon::createFromFormat('Y-m-d H:i:s', $value)->format(config('project.datetime_format')) : null;
    }

    public function getDeletedAtAttribute($value)
    {
        return $value ? Carbon::createFromFormat('Y-m-d H:i:s', $value)->format(config('project.datetime_format')) : null;
    }

    public function team()
    {
        return $this->belongsTo(Team::class);
    }
}

what shall I do please ???