Uncaught ReferenceError: $ is not defined at Object.onClick

Question

i followed a video about integrating Django with PayPal, I did like he did exactly, with this code:

{% block script %}
    <script src="https://www.paypal.com/sdk/js?client-id=AcNrdRh7kSF3PCa6_eS3i0Hyg_EOiwOZ75NfQ0NZQVUpxZCpReVeRuvq28_jfiWzsutpBwNpC8gGnT2V&currency=USD"></script>

    <script>
        paypal.Buttons({
            //validate the payment form
            onClick: function (data, actions) {
                var fname = $("[name='fname']").val();
                var lname = $("[name='lname']").val();
                var email = $("[name='email']").val();
                var phone = $("[name='phone']").val();
                var address = $("[name='address']").val();
                var city = $("[name='city']").val();
                var state = $("[name='state']").val();
                var country = $("[name='country']").val();
                var pincode = $("[name='pincode']").val();
                var token = $("[name='csrfmiddlewaretoken']").val();

                if (fname == "" || lname == "" || email == "" || phone == "" || address == "" ||
                    city == "" || state == "" || country == "" || pincode == "" || token == "") {
                    swal("Alert !", "All fields are mandatory", "error");
                    return false;
                } else {
                    return true;
                }
            },

before adding this code, everything was working correctly, after this code shows me this message in browser console:

Uncaught ReferenceError: $ is not defined     at Object.onClick