If your are encountering this message it is likely a network issue, so you should first follow the steps in this KB article: License Error. The licensing servers are currently unavailable and your local key has expired.
However, it may be the result of your PHP time being greatly out of sync. You can check this by running a system health check by navigating to the following address /system/heath.php then logging in.
If you see PHP time messages and failure Checking HTTP/S access to MediaCP. Then running the following two commands should get this working for you again:
systemctl restart ntpd
Then a wait for a minute and run:
mediact restart php
