Hi Canuck
Most likely because (like almost every other version of MYOB since year dot) that version does not allow entries to prior year.
MYOB have been around for over 30 years and has never allowed this. Once you start a new year, the previous year is locked off. It is only in the last 2 years with the latest version in Aust that MYOB now allows entries to prior years. Things might be different in your region.
See current year setting under Setup > Company Information.
You would need to restore a backup, process, generate reports etc, them make compensating adjustments in your current file - best to discuss this with your accountant.
Regards
Gavin