TRANS_IN_USE: This transaction cannot be deleted because it is linked to by one or more transactions.
This error occurs when a bill was deleted in Bill.com, but cannot be deleted in NetSuite because it has payments associated with it.
Any associated payments in NetSuite need to be voided so the bill can be deleted. First, void/delete any payments associated with the bill in NetSuite, and then delete the bill itself.
Then, contact Bill.com Customer Support to mark the bill not to sync.