You account is likely at its maximum user count.
Each account has a defined number of Active users permitted.
Your options to fix this are:
- have your administrator purchase more users (firstname.lastname@example.org)
- In the Resources sidebar, delete unneeded users from the account
- In the Resources sidebar, mark other users as inactive who do not currently need access
Be aware that a deactivated user can be reactivated at a later date. However, a deleted user will be removed from all reviews, projects and deliverables on the account and anything assigned to them (Projects, Deliverables, ToDos) will be reassigned to the account owner.
Once enough users licenses are available for the number of users you are trying to activate they can be enabled and new users will be created in the Activity status you choose.