The RD Licensing grace period has expired and the service has not registered with a license server with installed licenses. A RD Licensing server is required for continuous operation. A Remote Desktop Session Host server can operate without a license server for 120 days after initial start up.
The official solution is to Activate the RDS/TS CAL License server and point the Server to License server with User/Device License and will be resolve the problem, but if you want to reset the timer and again avail the 120 days grace time here is the solution:
Delete the REG_BINARY in:HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server\RCM\GracePeriod
To delete the key you must take ownership and give admin users full control.
After a restart of the server RDS will reset the grace period to 120 days.