Each instance of PPO has a license key which allows for the number of subscriptions to be managed. The error message below indicates that the number of active users on PPO is currently the same as the licence key / subscription and thus the new user account cannot be added.
To resolve this issue there are two possible solutions:
1. On the Administration >> Users >> List page check the current Active users and mark one of the currently Active users as Inactive (click on the user record and untick the Active flag). The number of Active users will thus be one less than the license key / subscription and the new user can now be added.
2. Alternatively you can request an increase to your subscription, as defined in this FAQ.