This article is to guide you on what to do when faced with the following error.
'Required parameter LinkedTxn.TxnId' is missing in the request'
Mark the shift as synced (Mark Synced).
This error would likely happen due to a dummy/test transaction that was reset previously, but the iPad was not deactivated (because deactivation of the Register is required after resetting transactions on the Register).
The shift would have have likely remained open but since the transaction was already removed from the system due to the reset of test transactions, it's okay to Mark Synced if it's a test transaction.