It is because you are not a premium member. Chess Mentor is a premium feature. See http://www.chess.com/membership Platinum membership is the only one which gives you unlimited Chess Mentor.
When you could do as many as you wanted, you either had a premium membership which you did not renew or you had a trial premium membership.
... or you happened to only be doing the set of free Chess Mentor lessons mentioned in this blog post http://www.chess.com/blog/webmaster/free-chess-mentor-courses
... or you happened to only be doing the set of free Chess Mentor lessons mentioned in this blog post http://www.chess.com/blog/webmaster/free-chess-mentor-courses
Thank you very much.Very useful.
I just selected a lesson and finished it . Before this i can do as much as i want.How can i recover it ? one lesson /day is not enough.