This happens when ConnectBooks deletes a settlement from QuickBooks Online, but one of its transactions is matched in Banking, or is on a deposit. QuickBooks won't let ConnectBooks remove it, and the settlement shows "Error Deleting".
How to fix it
In QuickBooks Online, go to Banking and unmatch the transaction. If it's on a deposit instead, remove it from the deposit.
In ConnectBooks, go to Accounting Export. Click the 3 dots on the settlement and choose "Delete from QuickBooks" again. If you see Settlement, COGS and Both, pick the one that shows the error.
Frequently asked questions
Why didn't I see this right away?
ConnectBooks removes settlements from QuickBooks in the background, so the error can show up after you click.