That error means that the cPanel license has been suspended because the server has too many users.
You can find out what kind of license you have by putting your server's IP into the following form:
If you pay cPanel directly for your license, reach out to cPanel customer support for assistance with adding users to your license.
If you are paying a different company for your cPanel license, reach out to the customer support for that company.
The company that you need to talk to will usually be listed on the verification page I linked to above.