To fix this, you will either have to:

1) Buy new users or

2) Disable the current users

However, there are instances when you may encounter this issue even if you haven’t reached the maximum number of System Users permitted by your subscription plan. For instance, let’s say your plan allows for 5 System Users, and you’ve only created 3 users. Yet, when attempting to create the 4th user, you receive the aforementioned message. This can occur if some users have a value greater than 1 in the “Simultaneous Sessions” field.

Each simultaneous session is treated as 1 System User.

To resolve this and allow the system to permit the creation of more users, you need to decrease the number of simultaneous sessions for these users. To do this, navigate to the User’s profile, and under Security Settings, adjust the “Simultaneous Session” setting accordingly.