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To send data from BQE CORE to QuickBooks Online, or vice versa, you need to make sure that root or underlying data has been sent before trying to send the related data. In this case, the invoices cannot be sent to QuickBooks Online unless the related projects have been sent or synced. Similarly, you cannot send time entries unless the related employees and activity items have been sent first. That is why we recommend syncing data in the order they appear in the sync menu. Check CORE Help Center for details.
Alternatively, if you are unable to send invoices from BQE CORE to QuickBooks and get an error, it might be because the invoice number is already in use in QuickBooks. Try using a different invoice number and then sync.