I’ve tried to figure it out but I can’t find any error logs anywhere on the site
There are at least three locations where errors get logged:
You server's (Apache's) log. On plain Debian/Ubuntu systems that's /var/log/apache2/error.log. On a web hosting it depends on where the hoster provides it. Usually right next to access logs, but I've seen hosting where it's simply turned off (ouch!).
thirty bees' log. In log/ in the root of each shop installation. In a properly working shop, nothing appears there. On errors, log files appear.
Using Firefox, hit F12 to open a developer console, choose the Network analysis tab, reload the page, and you see each request listed as it get sent and the answer comes in. There should be one with a violet 500 badge. Click on it and see what was requested and what the answer was. Even if you don't understand the voodoo provided there, you can at least see which request went wrong.
In some cases it helps to turn off debug mode. Code not PHP 7.2 safe can respond to Ajax requests not with JSON, but with an error message instead. Applies when running PHP 7.2, only.