You do not say what is giving you the error.

Your code,

 public function cart() {
        
        if (Request::isMethod('post')) {
            $id_product = Request::get('id_product');
            $product = Product::find($id_product);
            Cart::add( array('id' => $id_product, 'name' => $product->name, 'qty' => 1, 'price' => $product->price) );
        }

        $cart = Cart::content();
        
        return view('cart', array(
            'cart' => $cart,
            'title' => 'Welcome',
            'description' => '',
            'page' => 'home')
        );
    }
    
    <form method="POST" action="{{url('cart')}}"> 
        <input type="text" name="product_id" value="{{$product->id}}"> 
        <input type="hidden" name="_token" value="{{ csrf_token() }}"> 
        <button type="submit" class="btn btn-primary"><i class="fa fa-shopping-cart"></i> Add to cart </button> 
    </form>

I can only see one thing that is not specified in the return view statement, {{$product->id}} but it is in the form code? is that suppose to be $cart->id?