Had the same problem, along with a few others. Seems theres a few teething problems with trac install, I've reported what I've found to support.
I've managed to get a rather kludgy workaround in place for now by doing the following:
1) Editing .htaccess and added basic authentication, pointing AuthUserFile to the repository's .passwd file
2) Ran trac-admin and added relevent permissions to an account, then was able to use web admin within trac on that account.
It's not the best way (uses basic http auth, no anonymous users and cannot logout without destroying session), but it works for me until the issues with trac can be resolved and picks up the users from the repository fine.
The issue with trac admin reporting no VERSION usually means that you didn't get the path to your trac env correct.
trac-admin /path/to/your/trac/ permission add some_username TRAC_ADMIN
Where path is the location of your web accesible trac env and username is the user to give the permissions to.
Like I said, a horrendous kludge but it's got me up and running until something better comes along