QuickBooks Online didn't accept a settlement because a document with the same reference number is already in QuickBooks.
Why it happens
When ConnectBooks sends a settlement as a journal entry or an inventory adjustment, it gives it the settlement's CB ID# as its number. You can see the CB ID# in the CB ID# column on the Accounting Export page. If QuickBooks already has a document with that number, it rejects the new one.
How to fix it
Look in QuickBooks Online for the document with the settlement's CB ID# as its number.
Pick one:
If that document is an old copy of this settlement, delete it in QuickBooks Online.
If it's a different document you want to keep, stop ConnectBooks from using the CB ID# as the number. Go to Stores & Files and click the store. On the Accounting tab, under Accounting Settings, click Settings. On the QuickBooks tab, click Configure Advanced Settings. You'll see a caution note. Change only this setting: tick "Do Not Use Group ID as Journal Ref", then click Save.
Or turn off "Custom transaction numbers" in your QuickBooks Online settings.
In ConnectBooks, go to Accounting Export. Click the 3 dots on the settlement and choose "Resend to QuickBooks". If you see Settlement, COGS and Both, pick the one that shows the error.
Frequently asked questions
I don't see "Do Not Use Group ID as Journal Ref".
It only shows for stores that send settlements as journal entries.