$wpdb working with ajax but showing ajax error instead of success

The issue in your code is simple: if you want to send a JSON response, then make sure it’s a valid JSON response, i.e. send the correct Content-Type header and a proper JSON-encoded body. With jQuery.ajax(), though, you can omit the header, but you’d need to set the dataType to json.

function my_ajax_callback1() {
    // Invalid JSON response.
    die();
}

function my_ajax_callback2() {
    // Good JSON response with a Content-Type header.

    header( 'Content-Type: application/json' );
    echo json_encode( array( 'update' => true ) );

    wp_die(); // die() works, but wp_die() is better
}

And actually, in your callback, you could simply use wp_send_json() like so:

function my_ajax_callback3() {
    wp_send_json( array( 'update' => true ) );
    // No need to call wp_die(), die() or exit().
}

And there also wp_send_json_success() and wp_send_json_error(), but in your JS, you would need to use data.data.<key> and not data.<key>, except for data.success.

function my_ajax_callback4() {
    // ... your code

    // In your success( data ) JS, you'd access the 'update' value via data.data.update

    if ( $result ) {
        wp_send_json_success( array( 'update' => true, 'msg' => 'Success!' ) );
    } else {
        wp_send_json_error( array( 'update' => false, 'msg' => 'Foo error' ) );
    }
}

So be sure to return a valid response data and although not required, I suggest you to consider creating a custom REST API endpoint for your custom AJAX action and point your AJAX requests to that API endpoint — which (by default) always returns a JSON response.

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