Can anyone help me in fixing the issue (terminating the instance due to error 474) and then Instance terminated by PMON
I can't see your trc file to confirm, but it sounds an awful lot like this

https://support.oracle.com/knowledge/Oracle%20Database%20Products/1361872_1.html

Instance Terminated With Error ORA-00474: SMON Process Terminated With Error (Doc ID 1361872.1)
Last updated on JULY 23, 2019


If you login to the MOS system, you'll see the the cause is due to a corrupted INDEX. A diagnostic and remediation script is provided to verify and fix the problem.

Any time you get a message like this or a ORA-600 message, the first place to go is My Oracle Support.