This information applies to MYOB AccountRight version 19. For later versions, see our help centre.
https://help.myob.com/wiki/x/rQKc
ANSWER ID:22430
If a purchase order contains a negative stock value, the following error displays when attempting to convert the order to a bill:
This prevents the bill from being recorded due to the negative quantity. Note that this issue only affects purchase orders with a negative stock quantity.
Workaround
Create the bill from scratch. Be sure to choose Bill as the purchase type.