Cannot save value using ajax in laravel

I'm using laravel and trying to save data using post through ajax but data is not saved in database. I'm getting following error: jquery.min.js:2 POST http://localhost:8000/admin/products/attributes/add 500 (Internal Server Error). My code is as follows:

view:

<script>
     $("#add_attributes_info").click(function(e){
        e.preventDefault(); 
        $.ajax({
            type: "POST",
            url: '/admin/products/attributes/add',
            data: $('#frmattributes').serialize(),
            success: function(msg) {
                console.log('success'+msg);
              }
        });
        });
</script>
 <form action="#" id="frmattributes" method="POST">
            <h3 class="tile-title">Add Attributes To Product</h3>
            <div class="row">
                <div class="col-md-4">
                    <div class="form-group">
                        <label for="values">Select an value <span class="m-l-5 text-danger"> *</span></label>
                        <select id="attribute_values" name="value" class="form-control custom-select mt-15">

                        </select>
                    </div>
                </div>
            </div>
            <div class="row">
                <div class="col-md-4">
                    <div class="form-group">
                        <label class="control-label" for="quantity">Quantity</label>
                        <input class="form-control" name="quantity" type="number" id="quantity"/>
                    </div>
                </div>
                <div class="col-md-4">
                    <div class="form-group">
                        <label class="control-label" for="price">Price</label>
                        <input class="form-control" name="price" type="text" id="price"/>
                        <small class="text-danger">This price will be added to the main price of product on frontend.</small>
                    </div>
                </div>
                <div class="col-md-12">
                    <button class="btn btn-sm btn-primary" id="add_attributes_info">
                        <i class="fa fa-plus"></i> Add
                    </button>
                </div>
            </div>
            </form>

Controller:

  public function addAttribute(Request $request)
    {
        $productAttribute = ProductAttribute::create($request->data);

        if ($productAttribute) {
            return response()->json(['message' => 'Product attribute added successfully.']);
        } else {
            return response()->json(['message' => 'Something went wrong while submitting product attribute.']);
        }
    }

via Chebli Mohamed