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.
Resolution:
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.
Return to your QuickBooks Online account and choose > Sales > invoices from the left navigation bar.
3. Randomly choose any one of the invoices
4. In your browser's internet address bar substitute the number after the equal sign
with the import job number from the Kickserv error message. This will locate the
QuickBooks job that is in conflict with your Kickserv job.
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 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.