What does this ERROR message mean?

The error message is specific to incremental repairs and is instantiated from the LocalSessions class of the repair service. This portion of the code was updated per CASSANDRA-9143 to avoid mixing repaired and unrepaired data in incremental repairs.

Why does this ERROR occur?

The error occurs as a result of try/catch block, in two scenarios:

            setStateAndSave(session, FINALIZE_PROMISED);

            sendMessage(from, Message.out(FINALIZE_PROMISE_MSG, new FinalizePromise(sessionID, getBroadcastAddressAndPort(), true)));

Either the session is not available or the sessionID is invalid.

How do you fix this ERROR?

If you are receiving this error, please contact Support to help diagnose the issue.