Oracle® Database 11g / 12c / 18c error codes ORA-00407: rolling upgrade from release [string].[string] to [string].[string] is not allowed

ORA-00407: rolling upgrade from release [string].[string] to [string].[string] is not allowed

Oracle Error Code: ORA-00407

Cause: Another instance executing software at a different point release already has the database mounted.

Action: Shutdown all instances then startup with the new software.