This error often happens when you try to sync a supplier credit note and is related to an inventory tracking item.
A tracked item has it inventory value tracked by MYOB
For example,
Let's say we buy 2 items AAA from a supplier with a price of 100, and A is an inventory tracking item.
Its inventory value thus will be $200
when you buy, sell, return the item. MYOB will track the quantity of the item as well as the value of the inventory.
For example, if you sell one item. its quantity will be 1 and the inventory value will be $100
When you enter a supplier credit note, which means you return the item to the supplier. If for example, on the supplier credit note you specify that you return the remaining item but the unit price of the item on the supplier credit note is 110 which means the value you return to the supplier is $110. MYOB will try and adjust the inventory value of the item to be 100 - 110 which is -10.
You have no item left in your inventory, but the value of the item is -10. MYOB will not accept this. you will have to check the item, average buy price, and the credit note to see where and how it has gone wrong.
Please be noted that all of this happens in MYOB. MechanicDesk has nothing to do with it. It simply trying to push the supplier credit note you specify in MechanicDesk to MYOB. MYOB is the one that track the inventory and policing rules of inventory tracking stocks.