If you are seeing the following error in the platform when using QuickBooks Desktop:
ERROR: Error from QuickBooks: There was an error adding, modifying or deleting xxx-12345678910 because it is already in use. QuickBooks error message: The transaction could not be locked. It is in use by another user.This is due to invoice(s) being currently opened by another user.
The first step is to see if other users in QuickBooks are editing transactions. If so, request that they complete their editing. Once the edits are saved, try to sync again.
If you continue to see the error, switch the company file to single-user mode, close QuickBooks, and sync again.
If after confirming that no one is using the transaction, the sync still returns the error, run a sync in while in Single User Mode in QuickBooks. This should ensure that no other files are being used outside of your workstation. Should the error pop up again after this, that means that the transaction file in question is hung up on your system. To fix this, reboot your workstation and do another single-user sync.