Error: Insufficient User Licenses
You are getting this error because there are more registered users than slots allocated within your SharePoint Domain.
Users are cleared from your account at the start of the month, so each month starts with a fresh count. You can set your monthly reassignment to commence with a smaller or larger number of users. You can only adjust the user count down at the beginning of a new month prior to the user cache clearing, which happens directly before your new month roll-over.
Your administrator needs to increase the license count for the SharePoint Domain that is receiving this message. Your administrator can increase the User Count at any time.
Increase your user license count
Schedule to decrease your user license count