defect submit-button after forgetting checkbox

Hi,

i’ve an issue on my reservation page using the visual form builder. On the front clients have to fill in their name, emailadress and phone number. but when they forget to check the checkbox “algemene voorwaarden” and they yet check the submit button “ga verder” it won’t work and doesnt goes to the thankyou page.

can you help me out?

thx

http://www.massagepraktijkhasselt.nl/afspraak-maken/
(you can make a test reservation if you want)

https://ww.wp.xz.cn/plugins/appointment-booking-calendar/