ORA-16647
maximum number of observers has already been started
Cause
The attempt to start an observer failed because the maximum number of observers was already running. For Oracle Database releases earlier than 12.2, only a single observer can be started. For Oracle Database releases 12.2 through 19c, a maximum of three observers can be started. For Oracle Database release 21c and later, a maximum of 4 observers can be started.
Action
Stop one of the existing observers and retry the command.