This invoice sync error occured due to a line being deleted from the invoice after it was matched with a purchase order.
Firstly, the invoice should be reset in SageAPA. The invoice can then be found in the In-Progress tab.
Ensure that the invoice is matched to the correct purchase order. Un-match the purchase order and match to the correct purchase order if the invoice was initially matched to the incorrect purchase order.
However, if the invoice was matched to the correct purchase order, we must adopt all lines found in the purchase order to the invoice before exporting. Refer to the matched purchase order and add any missing lines to the invoice. Once added, the invoice can be exported and synced into Sage 100.
A little advise:
Users should not delete lines from an imported purchase order or a purchase order matched invoice. If the line is not important, change the items of that line to 0 rather than deleting the row entirely.