The 500 Internal Server Error is a very general HTTP status code that means something has gone wrong on the website's server, but the server could not be more specific on what the exact problem is. They are often caused by plugin or theme function conflicts, or by configuration issues from your hosting side.
First of all, make sure that your website uses PHP 5.6 or higher. PHP 5.6 is the minimum required by ShortPixel. You can check it with a plugin like this one. If it isn't, please contact your hosting provider.
Now, you can try these options, in order:
- Update all plugins (make sure you have a backup of your site first!). There's a chance you have a problematic plugin that will solve the issue by just updating it.
- Try deactivating all plugins and installing ShortPixel. Does it work? If so, activate one by one the rest of the plugins until you find the one causing problems.
- Change the theme (you can switch to Twenty Nineteen, the default WordPress theme) and see if ShortPixel installs now. If it does, please contact the author(s) of the theme you were using, so they can solve this issue.
- If that does not resolve the issue, it’s possible that a
.htaccessrule could be the source of the problem. To check for this, access your server via FTP, or a file manager in your hosting account’s control panel, and rename the.htaccessfile to whatever you like (don't delete it, just rename it so you can restore it if needed). If you can’t find a.htaccessfile, make sure that you have set your FTP client to view invisible files.
If that doesn't help, we'd suggest contacting your hosting provider. They may be able to see more error messages or have a better understanding of the error.