Why PostgreSQL can't create user?
There must be a lock on a system catalog preventing the update. This is a pretty unusual occurrence.


Check pg_stat_activity for long running transactions that are not in state idle. Cancel those.
Look for prepared transactions in pg_prepared_statements and roll them back.


If both don't help, have a look at pg_locks while your CREATE USER hangs. It will show you for which transaction the statement is waiting.