Oracle Message ORA-16773 - cannot start Redo Apply

ERRORcannot start Redo Apply
CAUSEThe Data Guard broker failed to start Redo Apply.
ACTIONCheck the Data Guard broker log and Oracle alert log for more details. If the database is already open, the broker will not be able to start Redo Apply if the COMPATIBLE initialization parameter is set to a value less than 11.0.