Undefine index in laravel 5.8

      $random_sku= Str::random(10);

        $productDetails= Product::with('attributes')->where(['id'=>$id])->first();
        
        //$productsDetails= json_decode(json_encode($productsDetails));
        
        if($request->isMethod('post')){
            
            $data=$request->all();
            
            foreach($data['product_sku'] as $key=>$value){
                
                if(!empty($value)){
                    
                    //Prevent SKU Duplication SKU Check
                    
                     $attCountSKUSeller=ProductAttributes::where('product_sku',$value)->count();
                             
                     if($attCountSKUSeller>0){
                         
                         return redirect('/seller-add-product-attributes/'.$id)->with('flash_message_error','product SKU already exists, please add another Product SKU!');
                     }
                     
                     //Prevent Size Duplication Check
                     
                     $attrcountSizesSeller= ProductAttributes::where(['product_id'=>$id, 'product_size'=>$data['product_size'][$key]])->count();
                     
                     if($attrcountSizesSeller>0){
                         
                          return redirect('/seller-add-product-attributes/'.$id)->with('flash_message_error','"'.$data['product_size'][$key].'" Size already exists for this product, please add another Size!');
                     }
                     
                    $attribute=new ProductAttributes;
                   
                     
                    $attribute->product_id=$id;
                   
                    $attribute->product_sku=$value;
                    
                    $attribute->product_size=$data['product_size'][$key];
                    
                    $attribute->product_price=$data['product_price'][$key];

                    $attribute->product_colour=$data['product_colour'][$key];

                    $attribute->product_stock=$data['product_stock'][$key];
                    
                    $attribute->save();
                            
                }
            }
            
            return redirect('/seller-add-product-attributes/'.$id)->with('flash_message_success','Product(s) Attribute(s) have been Added Successfully');
        }
    
        
         return view('shop.seller_product_attributes')->with(compact('productDetails', 'random_sku'));

     //return view('shop.seller_product_attributes');

}

pls, guys help me with this problem I have tried numerous laravel helper function like create(); insert();.I don’t know what i am doing wrong and if there is better way of doing this help me.these is my view:

Price
              <div class="col-md-2 col-sm-2 col-lg-2 col-xs-2">
                  <label for="product_size">Size</label>
                  <select class="browser-default custom-select custom-select mb-3" name="product_size[]" id="product_size" >
                 <option selected disabled>Select Size</option>
                  <option>Small</option>
                  <option>Big</option>
                 <option>Extra Large</option>

              </select>
                  </div>
              
               <div class="col-md-2 col-sm-2 col-lg-2 col-xs-2">
                    <label for="product_colour">Colour</label>
              <input type="text" name="product_colour[]" id="product_colour" placeholder="Colour" class="form-control mb-3" validate="name" >
              </div>
              <div class="col-md-2 col-sm-2 col-lg-2 col-xs-2">
                    <label for="product_stock">Stock</label>
                 <input type="number" name="product_stock[]" id="product_stock" placeholder="Quantity" class="form-control mb-3" >
              </div>
             
             
              <div class="button-group mt-4 my-2">
                  <a href="javascript:void(0)" class="btn btn-primary  btn-sm" id="plus5">Add More</a>
                  <a href="javascript:void(0)" class="btn btn-danger btn-sm" id="minus5">Remove</a>
              </div>
          </div>
        </div>
        <div class="row d-flex justify-content-center ">
  <button type="submit" class="btn btn-primary btn-block text-center mb-4 mx-2">Submit</button>
  </div>
    </form>

what I am trying to do is multiple insertions

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