Your QuickBooks Online subscription has run out or wasn't paid. Until it's active again, QuickBooks won't accept anything from ConnectBooks.
How to fix it
Log in to QuickBooks Online as the main admin.
Click the gear icon, then Account and settings, then Billing & subscription. Some accounts call it Subscriptions and billing.
Click Resubscribe, or update your card.
In ConnectBooks, go to Accounting Export. For each settlement that shows "Error Syncing", click the 3 dots and choose "Resend to QuickBooks".
Frequently asked questions
I renewed, but I still see the error.
Settlements that failed while QuickBooks was expired don't resend on their own. Resend them as in step 4.
Will I lose any data?
No. ConnectBooks keeps bringing in your settlements. The ones that weren't sent yet go out by themselves after you renew. The ones that show "Error Syncing" need to be resent.
My free trial ended.
That counts as expired. Choose a paid QuickBooks plan, then resend.