@M_iA Ah! you are right your GlideAjax is an asynchronous call and your form is already getting submitted before you get the response from the server.

Please update your client script based on the following code.

function onSubmit() {
if (g_scratchpad.isFormValid){
	return true;
}
    var checkboxResponse = g_form.getValue('is_employee_available');
    if (checkboxResponse == true || checkboxResponse == "true") {
       var actionName = g_form.getActionName();
        var ga = new GlideAjax('global.CustomerHelpCenterUtils');
        ga.addParam('sysparm_name', 'insertUserRecord');
        ga.addParam('sysparm_user_first', g_form.getValue('first_name'));
        ga.addParam('sysparm_user_last', g_form.getValue('last_name'));
        ga.addParam('sysparm_user_email', g_form.getValue('email_address'));
        ga.addParam('sysparm_user_telephone', g_form.getValue('telephone'));
		ga.addParam('sysparm_user_company', g_form.getValue('account'));
		ga.addParam('sysparm_user_id', g_form.getValue('employee_id_number'));
        ga.getXMLAnswer(myCallbackFunction);
    }

    function myCallbackFunction(response) {
                g_scratchpad.isFormValid = true;
	g_form.submit(actionName);
		var answer = response;
		alert(answer);
		g_form.setValue('new_contact_record', answer);
    }
return false;
}

Source: https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB0783579

Hope this helps.