Q: When I do File > Backup from the RTG Bills menu, I get the error message Permission denied and the backup stops.
A: If you are writing to a removable medium, check if it is write-protected. On a 3.5" floppy disk, the hole on the right side should be covered.
If the destination disk is not the problem, this message usually means that you were (perhaps accidentally) running two copies of RTG Bills.
RTG Bills is a multi-user program, which means that normally several people can run it at the same time. However, that isn't true during a backup. Only one copy of RTG Bills can be running. (If another copy of RTG Bills were running, it could modify the database during the backup, which creates problems.)
Occasionally Windows gets confused about whether a file is "in use" or not. Shut down and restart Windows to fix that.
Note that running RTG Timer does not conflict with an RTG Bills backup.