ORA-16677: physical standby database cannot be an alternate destination
Cause:
A physical standby database was set as an alternate destination for another physical standby, or for a far sync instance and the FALLBACK keyword was not specified.
Action:
Ensure that a physical standby database is not set to be an alternate destination for another physical standby. If the physical standby is to be an alternate destination for a far sync instance, be sure to specify the FALLBACK keyword.