I have already tried that there is something more wrong with portal for some reason but I think it is limited to the implementation in Suitecrm as it does not happen on a standalone installation.
If you remove the logic which checks if the user has already been enabled for the portal. The option does appear but if then you click it, the process error’s.
If I knew I wasn’t going to cause any issues, I would install the standalone module on top which resolves the issue.
Just to inform you if you did not already know - SuiteCRM 7.0.1 is out. This features many bug fixes including addressing the Create Portal User issue and some UI changes. See the downloads section to get the SuiteCRM 7.0.1 upgrade.