There is a feature that can be activated on the backend of your Kickserv account. That feature will force the Kickserv job number to become your QuickBooks invoice number. If a QuickBooks invoice number is the same as an existing KSA job number a sync error will occur.
1. In Kickserv locate your last or highest job number. If you don't know it you could temporarily create a test job and see what number it becomes. Make note of this job number and delete the record.
2. Return to your QuickBooks Online account and choose Sales invoices from the left navigation bar.
5. Make note of this QuickBooks invoice
6. Navigate to the QuickBooks settings and choose Account & Settings
7. Click on Sales Custom transaction number.
8. Check the box to turn custom transactions ON and Save the update and then Done.
9. In QuickBooks search for the invoice number you recently noted
10. The invoice box in the invoice will now be replaced with the next highest Kickserv job number you discovered in step #1 and save updates.
11. Follow the steps back to Account and Settings and turn custom transactions OFF again.
12. Return to your Kickserv account and run the global sync once more.