Web Form Validation Script not working properly (V14)

Dear community,

I am testing web form in V14. What I am testing is pretty simple.

I have a form with 1 check box, and if the box is not checked, it should show error message. Here is the script I have on the Client Script section of the web form.

frappe.web_form.validate = () => {
    let data = frappe.web_form.get_values();
    if (!data.checked) {
        frappe.msgprint('No Checked');
        return false;
    }
    return true;
}

The validation is checked, but the message do not persist on screen. Anything is incorrect here?

web_form_validation