Continue button sends me to same page

[kalcable]

Hello,

 

I've installed a new contribution and when I click Continue when checking out (to confirm order) it just sends me to the same page I was before. How can i make it so that continue button sends me to the next page. The contribution is Simple Checkout for 2.3.1. I tried to go on their support page, but no one seems to be active on that thread anymore.

 

 

Thanks!