Hi,
I tested on your website but I do get the popup with the terms and conditions on your checkout:
monosnap.com/file/CDf3keSj1eZ4zRMUpfy3febit4HtPj
You just have to enter the text for it.
Regarding the 500 error, I see this happens a long time after the button is clicked, like 4 minutes after that.
Do you get the order created in the backend orders listing ?
If yes, it could be linked to the email sending process which cannot establish the connection to the email server.
Otherwise, I believe it's linked to another extension, probably a plugin of the group "hikashop" or "system".
So try disabling them one by one via the Joomla plugins manager and try each time to see if you get the checkout to finish properly.
That way, you can identify the plugin causing the issue.