There is currently a general issue with the PTC License Server.
It is NOT something that you can solve at your own.
This issue has been resolved as indicated by Michael Siegenthaler on this discussion: PTC License Server for Creo Elements Direct Express dead/removed?
I don't know how anyone can refer to a recurring problem as solved. Again this month Modelling express d=fails to find the server and asks for reactivation, That fails and it asks for a 72 hour key. Press the button for key and get a code 4 times. Every time the key you entered is not valid.
Please, please, please resolve the issue or change your licence terms. I would pay a token amount from my old age pension to continue using Modelling express.