Hello,
Thank you for posting in the Microsoft community forum.
It looks like you're having a problem entering the license code. After the entered License Code is correct, you are advised to rebuild the license server database: Open Remote Desktop Authorization Manager. Right-click the server and select Manage Licenses. Select Rebuild License Server Database. Please refer to this article for more details: Remote Desktop Licensing Service Stopping - Microsoft Community Hub
If the problem persists, it is recommended that you contact a Microsoft licensing expert directly for help.
I hope this helps.
Best regards
Jacen