Hmm this is very old code... Do you have a "admin" group? and if you do, are your user a member of this group??
Note : This code shouldn't be used, as you should manipulate user group and membership with auth.add_group(), auth.add_membership(), auth.del_membership() and all others auth methods related to user management. There is many advantages to do so like making sure proper live variables values get updated properly. Other then that you may require to logout and login again to force those variables to be updated.
You may consider improving the Bruno's code by replacing SQLFORM() by proper auth methods.
Thanks
Richard