Fatal Error refers to ComposerAutoloaderInite

This site is currently in Maintenance Mode and requires admin access to see the error. The issue happens when I try to activate the Pro version of the plugin. I do have access to the client’s Pro account. I looked for a solution on Stack Overflow, and someone responded like this:

I had this issue when including two composer autoloaders from two directories where one originated as a copy of the other one (don’t ask me why).

If you can assist, that would be great. Many thanks for the support.

The page I need help with: [log in to see the link]