Project Server 2013 - _canBeAssignedIsEnabled error when adding a new resource

So consider the following scenario:

- You've recently applied a cumulative update to Project Server 2013 & SharePoint

OR

- You've imported an instance of Project Server from a farm that was at an earlier cumulative update version of Project Server, and it's showing that the upgrade part of the migration process was successful.

Then you go to add a new user to that Project Server instance (in Project Server mode, not SharePoint user mode).  When you add the user, you get an error that says _canBeAssignedIsEnabled does not exist.

The typical reason for this is you've applied the patch to your farm, but you've not actually upgraded the instance.  You can check the Project Server instance status by running the following Powershell:

Get-SPProjectWebInstance -Url http://server/pwa

If NeedsUpgrade = True, then you can run the following Powershell to upgrade the instance:

Upgrade-SPProjectWebInstance -Identity "http://servername/pwa"

We found that after running this, the NeedsUpgrade flag was changed to False, but we were still seeing the error.  It turned out that even in spite of running the upgrade command for the individual instance (which upgraded the content database), we still needed to run the Configuration Wizard for the farm as a whole.

After running the config wizard, we were able to add users in Project again.