Never edit superadmin.
Gmod expects a superadmin.
I've no clue what you edited to 'set to default', but you shouldn't be editing lua files to get rid of what Gmod expects as a group named superadmin to exist.
At best, someone here could have told you how to rename the superadmin group in data/Ulib/groups.txt back to superadmin.
Now, we only guess as to what you may have broken, and you may have made your issue worse.