Hi WaihiCH
The message refers to items with negative on hand quantities 'or have been received without a bill' - do you use MYOB's Receive Items feature?
To check, open Purchase Register, select Orders tab, set start date to 1/7/2000 - check for a tick in Received column - any there? Any dated before the proposed Locked Period Date? If yes, then these are what the message refers to. Suggest you spend some time in the Support section searching for 'Received Items' or similar for a better understanding of how this works.
Whenever you Receive Items against an Order, MUST convert that Order to a Bill.
If you do not use Received Items (that is, there are no ticks) - there may be a bug in your file that needs to be fixed. Suggest you send requested details to Shella_A
Regards
Gavin