Cause: The session specified in an ALTER SYSTEM KILL SESSION command cannot be killed immediately (because it is rolling back or blocked on a network operation), but it has been marked for kill. This means it will be killed as soon as possible after its current uninterruptable operation is done. See the solution below.

Error solution: No action is required for the session to be killed, but further executions of the ALTER SYSTEM KILL SESSION command on this session may cause the session to be killed sooner.