This took some time to figure out (found answer here)and it might come in handy sometime in the far future so I write it down here and now.
To create a user for the postgresql database on linux you have to do that from the linux user called
This would not work:
peterbe@computer:~$ createuser peterbe
What you need to do is this:
peterbe@computer:~$ su - ...log in as root... computer:~# su postgres computer:/root$ createuser peterbe
Now I know!