My guess is there was a bill for October and November time which you undid. You can redo this bill by putting an end date in the generate bills. You can change the bill date to be whatever date is appropriate. Once you generate and approve that bill you can do a December bill. I would suggest making a backup before you start.
If you would like some paid help/training you can contact me or one of the other consultants that participates in this forum (like Nancy Duhon) and we can help you directly with a remote session.

Thank you for the insight. It helped, but I'm still not getting the process completely. I keep trying to remove the one client from the revision stage, but get a message that all the other timeslips will be included.

I agree with Caren, you appear to have undone the October billing. You will need to REDO that bill before you will have an accurate November or December bill.

To redo that bill, you have to be a bit careful and select via a date range filter only the slips that should have been on that October bill, change the bill date to the date that bill originally went out, rebill it and reapprove it, then open up the date range to allow the newer slips to come in. Be careful that newer payments do not sneak in. This redo should match EACTLY as to the amount that was sent to the client in October so that their Balance Forward will be correct on the second bill.

What you have done is sort of "par for the course" for a typical user who has received no training on how to run the program. There are two types of mistakes that newbie's to the software make. 1) They UNDER approve the bills they produce as you have here. (Fail to approve when they should.) or 2) they OVER approve bills that they shouldn't, then later cannot undo them. Learning when you are ready to approve and should do so is key to using Timeslips effectively.

While I understand that the boss may not appreciate and have budgeted for training, there's really only so far that we can take you via this free bulletin board. Please read the included Help topic on Revision Stage from within the program to understand why the message is telling you what it is.