Oracle Solution for Error ora-12809

ORA-12809: cannot set string_INSTANCES when mounted in exclusive mode

What triggered the Error:

An attempt was made to set SCAN_INSTANCES or CACHE_INSTANCES using the ALTER SYSTEM command while the database was mounted in exclusive mode.

What should we do to fix it:

SCAN_INSTANCES / CACHE_INSTANCES may not be set unless running Oracle Real Application Clusters mounted in CLUSTER_DATABASE mode.