This folder was created before Catalina, probably as the result of mis-applying a Terminal command. In Catalina you are not allowed to create or delete folders at the top level of the system drive, so you’d have to jump through some hoops to delete that. You could reinstall like was suggested or you could just leave it be, since it is not used for anything.
For the record: ~admin (by itself) designates the home folder of the user called admin (in other words it is usually a shorthand for /Users/admin)
Some mistake led to /~admin (with the slash up front). As I said before this happened *before* Catalina.