The guest account is used internally by OMERO. It's best not to try to activate or otherwise modify the account. Sorry for the confusion.
If you would still like a guest-like account, perhaps you could let us know specifically what access this user should have, etc. and we can tell you the best way forwards. For example, you could enable a "public user" who can selectively be added to particular groups. See
"Enabling a public user" in the OMERO.web install guide.
Cheers,
~Josh