The 500 Internal Server Error is often caused by plugin or theme functions, but it’s exhausting the PHP memory limit in my case.

So, I need to increase the memory limit.
Here is how I increase the memory limit for my WordPress site:

Find the wp‑config.php file and search this comment line – That's all, stop editing! Happy blogging.

Then, add the following code above the comment line.

define('WP_MEMORY_LIMIT', '256M');
/* That's all, stop editing! Happy blogging. */

Then go to cPanel and look for MultiPHP INI Editor. Select a location on Basic Mode, change the memory_limit, and save.

This solution works for me!