Hi,

500 Internal Error is a generic error and says about some issue on the server and can be caused by different things. Anyway, to understand its nature you need to see your server error logs or enable PHP error display on the website. If you can’t do this, you need to contact your hosting provider for help in this question. Here is an article that may help you understand the error better http://www.wpbeginner.com/wp-tutorials/how-to-fix-the-internal-server-error-in-wordpress/

Could you please send us a link where we can see the problem with Frequently Bought Together products? Also, send us some screenshot so we can better understand your problem.

Regards