I am not getting success response after inserting/updating record to the database. Instead ajax returning error: function(response) with Something went wrong! message.

$this->gs_db is the database class instance and $this->gs_db->db is $wpdb

Class Method

/**
* Add product group into the cart
*
* @param int      $group_id      group id
* @param int      $product_group product group id
* @param int|bool $group_user    group user / logged in id
*
* @return int inserted row id if new else update row id
*/
public function add_pg_to_cart($group_id, $product_group, $group_user = FALSE)
{

        $this->group_id      = $group_id;
        $this->qty           = 1;
        $this->product_group = $product_group;
        $this->group_admin   = gs_get_group_admin($this->group_id);
        $this->group_user    = ($group_user) ? absint($group_user) : absint(get_current_user_id());


        // setup data to insert
        $this->data = [
            'product_group' => $this->product_group,
            'qty'           => $this->qty,
            'group_id'      => $this->group_id,
            'group_admin'   => $this->group_admin,
            'group_user'    => $this->group_user,
        ];

        // setup data format
        $this->format = [
            '%d',
            '%d',
            '%d',
            '%d',
            '%d',
        ];

        // insert / update query
        $table = $this->gs_db->get_pg_cart_table();
        $query = "INSERT INTO {$table} (product_group, qty, group_id, group_admin, group_user, added_at) 
                        VALUES('%d', '%d', '%d', '%d', '%d', now()) ON DUPLICATE KEY UPDATE qty = qty + 1, added_at = now()";

        $prepare = $this->gs_db->db->prepare($query, $this->product_group, $this->qty, $this->group_id, $this->group_admin, $this->group_user);
        $result  = $this->gs_db->db->get_row($prepare);

        // get the id for inserted record
        $this->id = $this->gs_db->db->insert_id;

        return $this->id;

}

Javascript

(function ($) {

    $(function () {

        let response_alert = $('#response');
        response_alert.hide();

        $('form.gsGpAddToCart').on('submit', function (e) {
            e.preventDefault();

            let formData = $(this).serialize();

            $.ajax({

                method: 'POST',
                dataType: 'json',
                url: ajax_vars.ajax_url,
                data: formData + "&action=gs_pg_add_to_cart&nonce=" + ajax_vars.nonce,
                success: function (response) {

                    console.log(response);

                    let response_alert = $('#response');

                    if (response.success === true) {
                        response_alert.fadeIn().prepend(alerts('success', response.data));
                    }

                    if (response.success === false) {
                        response_alert.fadeIn().prepend(alerts('error', response.data));
                    }

                },
                error: function (response) {

                    let error_message = (response.data === undefined) ? 'Something went wrong!' : response.data;
                    response_alert.fadeIn().prepend(alerts('error', error_message));
                    gs_alert_stats();
                },
                complete: function () {
                }
            });

            return false;
        });

        // control dynamically created error with JS - close/hide
        function gs_alert_stats() {
            let gs_alert = $('.gs-message');
            $('.gs-close').on('click', function () {
                alerts('Clicked');
                $(this).parent(gs_alert).fadeOut();
            });
        }

    });

})(jQuery);

Javascript is returning this error response Something went wrong! which is in error: function (response)