This error indicates that either 1) you have deleted a carry-forward credit in Recurly after it had already been synced to Quickbooks or 2) you have not yet enabled Credit Invoices in your Recurly site and therefore, credit memos are not being generated in order to sync invoices which have credits applied to them.
In the first scenario, you'd want to avoid deleting carry-forward credits when possible as we have no resolution for this otherwise. If you haven't enabled Credit Invoices and have invoices that have credits applied to them, then you may want to turn this feature on. Once it's on, credit memos and the invoices that have credits applied to them will sync over to QBO. Until then, you will need to manually add those invoices to QBO.