Ensure that the RDS License Server is configured properly through GPO on the VDAs as per CTX238259 . You can run RSOP to verify that it is being applied.
If the configuration is correct, check the following registry entry:
Path: HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlTerminal ServerRCMLicensing Core
The DWORD value needs to be 4 (if using Per User licenses) or 2 (if using Per Device Licenses)
If you find the value to be 5, change it to either 4 or 2 and start a new session to the VDA. A reboot is not usually required.