Laravel 5: Ajax Post 500 (Internal Server Error) Error

I'm trying to submit data via ajax (add to cart) but got an error in console POST http://localhost/glutax9gs/cartupdate 500 (Internal Server Error)

What's wrong with my code? Is it the javascript or the controller?

---

here is my Route:

Route::get('/cartupdate', 'FrontEndController@cartupdate')->name('update.cart');
Route::post('/cartupdate', 'FrontEndController@cartupdate')->name('cart.update');

Controller code is here:

    public function cartupdate(Request $request)
    {

        if ($request->isMethod('post')){

            if (empty(Session::get('uniqueid'))){

                $cart = new Cart;
                $cart->fill($request->all());
                Session::put('uniqueid', $request->uniqueid);
                $cart->save();

            }else{

                $cart = Cart::where('uniqueid',$request->uniqueid)
                    ->where('product',$request->product)->first();
                //$carts = Cart::where('uniqueid',$request->uniqueid)
                        //->where('product',$request->product)->count();
                if (count($cart) > 0 ){
                    $data =  $request->all();
                    $cart->update($data);
                }else{
                    $cart = new Cart;
                    $cart->fill($request->all());
                    $cart->save();
                }

            }
            return response()->json(['response' => 'Successfully Added to Cart.','product' => $request->product]);
        }

        $getcart = Cart::where('uniqueid',Session::get('uniqueid'))->get();

        return response()->json(['response' => $getcart]);
    }

jQuery code is here:

    $(".to-cart").click(function(){

        var formData = $(this).parents('form:first').serializeArray();
        $.ajax({
            type: "POST",
            url: '',
            data:formData,
            success: function (data) {
                getCart();
                $.notify(data.response, "success");
            },
            error: function (data) {
                console.log('Error:', data);
            }
        });
    });

View code is here:

        <form>
            <p>
                
                @if(Session::has('uniqueid'))
                    <input type="hidden" name="uniqueid" value="">
                @else
                    <input type="hidden" name="uniqueid" value="">
                @endif
      <input type="hidden" name="title" value="">
      <input type="hidden" name="product" value="">
      <input type="hidden" id="cost" name="cost" value="">
      <input type="hidden" id="quantity" name="quantity" value="1">
           @if($product->stock != 0 || $product->stock === null )
    <button type="button" class="button style-10 to-cart">Add to cart</button>
              @else
           <button type="button" class="button style-10 to-cart" disabled>Out Of Stock</button>
           @endif
            
            </p>
        </form>

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