This can have several causes.
1. Anther user already is connected to the fanuc robot instance (virtual robot).
2. RCS Server itself crashed -> restart the machine running the Fanuc RCS Server.
3. If Fanuc RCS Server is not installed on the same machine as PS, ensure the machine hosts Fanuc RCS Server has write access to the SystemRoot (robotsmachinedatafiles folder).
4. In your virtual robot folder there is already a "instance.cfg" file, which wasn´t delete previously because of any error. Delete this file and try to initalize again.