User gets an error when sending an invoice from BillQuick to QuickBooks.
It seems that the error occurs because this project exists in BillQuick but not in QuickBooks. This error is referencing a QBLinkID in the Project table of the BillQuick database that has no matching link in QuickBooks. This can happen if a project sent to QuickBooks was later deleted or merged there. According to the Project screen, the project is linked with QuickBooks. The QBLinkID still exists in the BillQuick database because it it does not know that the project no longer exists in QuickBooks.
You can resolve this issue in the following way:
1. Use the BillQuick-QuickBooks Smart Match utility to determine that the project is no longer linked with any records in QuickBooks. 
2. In the BillQuick-QuickBooks Integration Settings screen, select Send Settings panel>Projects.
3. Check 'Ignore Last Sync Date' and filter by the project record you want to send to QuickBooks (remember to undo these changes after the record is sent).
4. Send the project record as usual. Once the project is sent, you are able to sync subsequent records that reference it, such as time entries, invoices and payments.
You will now be able to send invoices for this project to QuickBooks without any error.