Oracle® Database 11g / 12c / 18c error codes ORA-12336: cannot login to database (link name [string])

ORA-12336: cannot login to database (link name [string])

Oracle Error Code: ORA-12336

Cause: You are tyring to login while another user is mounting or dismounting the same database.

Action: Check to see if the database is in the middle of being mounted or opened and try your login again once the database is accessible.