This problem happens because Windows only allows two remote terminal services connections when you are in administrative mode, and you’ve either got two people already on that server, or more likely, you’ve got a disconnected session that still thinks it is active.
Here’s a command line hack that you can use to figure out what sessions are connected to the server. Note that you could substitute the IP address for the server name.
query session /server:servername
Now we know that the session ID of the offending session is 2. We can use that in the next step, which is using the reset command to log off that user.
reset session [ID] /server:servername
Now you can try to run Remote Desktop again!