hmm... I don't believe that you can use the name "Admin" to name a user account on a Mac now. It is normally a name reserved by the system.
I could be wrong about that... But, if you go to the Users folder (at the root of your boot drive), is there a folder there named "Admin".
It's also possible that the upgrade to Monterey made that Admin folder invisible - so, in that Users folder, press the Shift, Option, and period (decimal point works too). You will see all the files and folders that are invisible on your drive.
Do you see a folder named "Admin" in the Users folder now?
If you want to hide all those files that are normally hidden, just press the same Shift-Command-. again.