Hello Kctrade
Thank you for attaching the error log for me to look into. Normally we would advise going off the Help Article for something has gone wrong and AccountRight needs to close, however, I can see the error log contans a different code than what is stated.
This code means you have selected to discount an item by 100%. What you will need to do is remove that item discount, convert the order and then re-add that discount once the transaction has been saved as an invoice/bill.
Additionally if you have the same item listed twice on the invoice it can cause this issue. so I would advise to join the item into a single line.
Please let me know how you go.