This can be of many reasons, and is quite simple to track down what happened.
1. Have a look at what MechanicDesk trying to push (from the error)
2. Then see which credit note it is coming from
3. then check the respective credit note in their MYOB (you might need Team Viewer) and see what happen with that credit note in MYOB that make the amount not enough.
E.g
Error at bill payment 1234 of amount 100, which come from credit note ABC
look at ABC in MYOB and check it total as well as allocation, it's remaining amount is less that 100 and we need to see why.
There are many cases I can think of why this happen.
1. They prematurely finalize the credit note in MD, then add more in MD, causing the total amount of the credit note in MD and in MYOB to be different.
2. They manually apply the allocation in MYOB which MD does not know and try to push the same allocation
3. The allocate in MD which sync to MYOB, then they remove the allocation in MD and allocate to a different bill in MD, MD try to push the new allocation but MYOB reject because the credit note has exhausted.