This can be caused by KC and QB losing connection when performing the sync. It can happen when syncing a long date range or a large number of invoices and receipts.
In KC, click on Utilities, QuickBooks DirectSync and do the sync again for this date and see if it will sync now.
Please note: KC will not resync any invoices/receipts that have already been synced to QB so it will try to sync only this item(s) that failed.