your page is missing the server validation server behavior.
go to Window -> Server Behaviors, then click the plus button and select WebAssist -> Validation -> Server Validation
for the validation type, select Like Entry. next to the server variable, click the lightning bolt and select the Registration_group_field form element, when you click the plus button to add it, you will be prompted to select the comparison variable, select the captcha_Registration_group_field_2 session variable.