Can you give me (super) admin and (s)ftp access so I can take a look?
And tell me on what url I can see/reproduce the issue.
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Weird stuff going on on your site.
I managed to get it installed by deleting the regularlabs plugin folder,
then uninstalling the Regular Labs Library and AMM via the Joomla installer.
Then re-installing AMM. It gave an error again. But after deleting the autoload_psr4.php file in the administrator/cache folder, it is working.
Well, not sure what's going on. But things are acting weird and not as they should do.
Maybe some other extensions getting in the way during installation (and you have a LOT of stuff installed).
But the fact that the admin cache file is not being re-generated on install is something that is not good.
Could be something left over from a migration from J3 to J4 that didn't go 100% ok. No idea.
The issues you are getting (with installing AMM not working and such) have nothing to do with my extensions and is not something I can fix for you.
Your setup has issues. See if the Database Fix thing solves it. Otherwise I recommend you ask on the general Joomla forum for further help.
forum.joomla.org