Laravel 10 Api, foreach array error

I am having this error "message": "foreach() argument must be of type array|object, string given", when i try to run my api products database

public function up(): void
    {
        Schema::create('products', function (Blueprint $table) {
            $table->id();
            $table->string('product_name');
            $table->string('product_code')->unique();
            $table->string('product_color')->nullable();
            $table->float('product_cost_price');
            $table->float('product_sales_price');
            $table->string('product_discount')->default(0);
            $table->longText('description')->nullable();
            $table->string('group_code')->nullable();
            $table->string('meta_title')->nullable();
            $table->string('meta_description')->nullable();
            $table->string('meta_keywords')->nullable();
            $table->mediumText('product_image')->nullable();
            $table->enum('best_seller', ['Yes', 'No'])->default('No');
            $table->boolean('status')->default(1);
            $table->integer('product_category_id')->foreign('product_category_id')->references('id')->on('product_categories')->onDelete('cascade');
            $table->integer('admin_created_id')->foreign('admin_created_id')->references('id')->on('users')->onDelete('cascade');
            $table->integer('admin_updated_id')->foreign('admin_updated_id')->references('id')->on('users')->onDelete('cascade')->nullable();
            $table->timestamps();
        });
    }```

product_attributes database

Schema::create('product_attributes', function (Blueprint $table) { $table->id(); $table->string('size')->nullable(); $table->float('cost_price'); $table->float('sales_price'); $table->integer('stock'); $table->string('sku')->unique(); $table->boolean('status')->default(1); $table->integer('product_id')->foreign('product_id')->references('id')->on('products')->onDelete('cascade'); $table->timestamps(); });```

Product.php Model

public function product_attributes()
    {
        return $this->hasMany('App\Models\ProductAttribute');
    }

ProductController.php file

use App\Models\Product;
use App\Models\ProductCategory;
use App\Models\ProductAttribute;
use App\Models\User;
use Auth;
use Carbon\Carbon;
use Illuminate\Support\Facades\Storage;
use Illuminate\Support\Str;
use App\Http\Controllers\Controller;
use Illuminate\Http\Request;

public function addProductAttribute(Request $request, $id)
    {
        $fields = $request->validate([
            'size' => 'required|max:255',
            'cost_price' => 'required|max:255',
            'sales_price' => 'required',
            'stock' => 'required',
            'sku' => 'required|unique:product_attributes,sku',
        ]);

        $product = Product::select('id','product_name','product_code','product_color','product_cost_price','product_sales_price','product_image')->with('product_attributes')->where(['id' => $id])->first();

        
        $data = $request->all();

        foreach($data['sku'] as $key => $value){
            if(!empty($value)){

                //Prevent SKU duplicate
                $skuCount = ProductAttribute::where('sku',$value)->count();
                if($skuCount>0){
                    return response()->json([
                        "message"=>"SKU Already exist!, Please add a new SKU"
                    ]);
                }

                //Prevent Size duplicate
                $sizeCount = ProductAttribute::where(['product_id'=>$id,'size'=>$data['size'][$key]])->count();
                if($sizeCount>0){
                    return response()->json([
                        "message"=>"Size Already exist!, Please add a new Size"
                    ]);
                }

                $product_attribute = new ProductAttribute;
                $product_attribute->product_id = $id;
                $product_attribute->sku = $value;
                $product_attribute->size = $data['size'][$key];
                $product_attribute->cost_price = $data['cost_price'][$key];
                $product_attribute->sales_price = $data['sales_price'][$key];
                $product_attribute->stock = $data['stock'][$key];
                $product_attribute->status = 1;
                $product_attribute->save();
            }
        }

        return response()->json([
            "product_attribute"=>$product_attribute,
            "message"=>"Product Attribute Added Successfully"
        ]);
    }```

The error is coming from foreach ``"message": "foreach() argument must be of type array|object, string given",``` an help?