No idea, however I faced the same problem. I must say it's stupid not allowing us to go back a move or two. When I'm playing against computer I want to have some fun, not try hard...
Why can't I move back all of a sudden against the computer?
On the app it still let me go back, just not on the pc site. You can sign out and it will let you go back.

Same problem!
Different problem cause, since this topic was from 2017, but it's being worked on. The made an update.
https://www.chess.com/forum/view/community/new-training-feature-practice#comment-61900255
I am a new and enjoy playing against the computer. I have been playing on this site for over a month and have always been able to go back as many moves as I want to redo a mistake. All of a sudden I am playing tonight and it will no longer let me go back. If I try it just automatically goes back to where I was. Anyone know why this is happening?