1) Firstly please open the Booking > Settings > Emails page and deactivate all email templates.
Then try to make new booking. If you will be able to make new booking, so then its means following:
Some severs can block sending emails and generate "500 Internal Error", when you send it to visitor from domain different from your website. You need to check, if this condition set in your server configuration.
- Please try to set in the field "From" instead of visitor email shortcode, your email (its at the Booking > Settings > Emails page). The same email that you set in the field "To".
- Sometimes mail servers block emails, if the address "From" different from the domain of website, from where this email was sending. Its not really depend from the Booking Calendar. So please open the Booking > Settings > Emails page and recheck that field "From", its have to the same as your website domain. For example: email@example.com and not like this firstname.lastname@example.org
Otherwise, please contact support of your hosting company about recheck conditions, when your server block sending of emails.
2) Such error can be because in your server configuration (probably at the WordPress > Settings > General page for the options "WordPress Address (URL)" or "Site Address (URL)") configured different Domain that you try to submit the booking
For example, in Ajax URL its can be set as "https://www.yourserver.com/wp-admin/admin-ajax.php"
and you try to submit the booking from https://www.yourserver.com/education/bookings/
Because booking is sending from HTTP to HTTPS, we are having such type of error.
Please recheck your server configuration at the Settings General page as in above description, or in some other place, where you are set using HTTPS instead of HTTP protocol.
You need to use the same protocol in both ways. If you will have difficulty with searching where exactly you are having this issue, please contact your server administrator who make that configuration.
The same issue can be if you configured sever address with "www." But trying to submit booking from URL without "www." prefix in URL. Or backward.
3) Also please try to deactivate one by one all your active plugins at the WordPress > Plugins menu. Then retest it again.
If its not help, please deactivate your active theme and active the default WordPress theme. And then retest it again. May be there exist some conflict with some active plugin or actual theme and just need to find the reason of that issue.
4) If the previous solutions will not help, please recheck your error.log file from your server configuration (please contact your server administrator or support of the hosting company, if you will have troubles with searching of this file) to be sure in a reason of that issue.