Feature test returning "role does not exists" with Spatie Laravel Permission

Quite simple, but not so simple. Here is the test that fails:

public function test_new_users_can_register(): void
    {
        $response = $this->post('/register', [
            'first_name' => 'Test',
            'last_name' => 'User',
            'email' => '[email protected]',
            'password' => 'password',
            'password_confirmation' => 'password',
        ]);

        $user = User::where('email', '[email protected]')->get();

        $user->assignRole('Admin'); // fails here

        $this->assertAuthenticated();
        $response->assertRedirect(RouteServiceProvider::HOME);
    }

Method Illuminate\Database\Eloquent\Collection::assignRole does not exist.

I have tried adding the HasRoles trait to the test class. User model also has the trait. Everything works fine on the frontend, but the testing environment is not recognising it. Any idea?

Full code -

namespace Tests\Feature\Auth;

use Tests\TestCase;
use App\Models\User;
use App\Providers\RouteServiceProvider;
use Illuminate\Foundation\Testing\RefreshDatabase;
use Spatie\Permission\Traits\HasRoles;

class RegistrationTest extends TestCase
{
    use RefreshDatabase, HasRoles;

	public function test_new_users_can_register(): void
    {
        $response = $this->post('/register', [
            'first_name' => 'Test',
            'last_name' => 'User',
            'email' => '[email protected]',
            'password' => 'password',
            'password_confirmation' => 'password',
        ]);

        $user = User::where('email', '[email protected]')->get();

        $user->assignRole('Admin'); // fails here

        $this->assertAuthenticated();
        $response->assertRedirect(RouteServiceProvider::HOME);
    }

namespace App\Models;

use Carbon\Carbon;
use Laravel\Sanctum\HasApiTokens;
use Spatie\Permission\Traits\HasRoles;
use Illuminate\Notifications\Notifiable;
use Illuminate\Contracts\Auth\MustVerifyEmail;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Foundation\Auth\User as Authenticatable;

class User extends Authenticatable implements MustVerifyEmail
{
    use HasApiTokens, HasFactory, Notifiable, HasRoles;

etc...