How do I grant PostgreSQL superuser access to vacuum tables?
It sounds like you revoked superuser privileges from the cluster's founding user.  Don't do that.  Log in as your remaining superuser, and grant superuser privileges back to the founding user.  If you have no remaining superusers, then you need to shut down the server, start it up in single-user mode, and regrant superuser privs.

The founding user is the username specified to initdb's -U argument when the database was created, or if that argument was not provided, then it is the name of the OS user who ran the initdb.  This is most often postgres.