Cause: An archive log needed by the standby database to resolve a redo gap was not found on the primary database. This is a fatal situation during a switchover because the primary database waits for the target standby to be clear of all archive log gaps before allowing the switchover to proceed. If a redo gap cannot be resolved, the switchover cannot succeed. See the solution below.

Error solution: If the specified log is listed in V$ARCHIVED_LOG for the DEST_ID of the local destination on the primary database but no longer exists in that location on disk, the archived log can be copied back to that location to allow gap processing to automatically resolve the gap. If the specified log has been purged from V$ARCHIVED_LOG, the archived log can be copied to disk on either the primary or standby database and registered using the ALTER DATABASE REGISTER OR REPLACE LOGFILE command.