Can't create new folders in Monterey

You can add folders anywhere inside your home folder, under /Users/Shared as already noted, and in other places like /usr/local - but you should only do that with good reason. The latter, and some others, is where many command line programs are installed. For anything that is user specific you should stay within your home folder.

I'm the Admin, so it should not be a question of Permissions...

Then you don’t understand the Unix permission model, along with the more advanced security model in macOS.

An admin user doesn’t convey omnipotent powers to that user. It merely allows the user to temporarily elevate their privileges to a higher level in order to take care of administrative tasks.

The closest thing to an omnipotent user that exists in Unix is root. However, Apple has severely curtailed root’s power with System Integrity Protection and the sealed, read-only System Volume.

You can make all the folders you want inside your home folder. There are a few other locations where you are allowed to modify, but if you just stick with your home folder, you won’t find any frustration.

You can add things to /Users/Shared (and probably /Users).

With Authentication, you can add folders to /Library, but that’s only useful if you need to add configuration files.

There are also some folders hidden in the Unix directories, but that whole area is hidden, so not particularly useful.

You may have been away for a long time.

Putting stuff at the top level of the hard drive has been recommended against for over twenty years; it has been "forbidden" for at least two years.

It was already not possible in Catalina, and much less so in Big Sur and Monterey, since now that area is part of a read-only volume; not even the OS or "root" can make changes, and that is a good thing. It makes the system much more secure.