Hello Manish Arora,

It seems like you have changed your Facebook password currently, so when you tried to submit the discussion post, then the post will trying to autopost in your Facebook, when the system can't able to access in your Facebook account then hitting this following error.

500 - Error validating access token: Session does not match current stored session. This may be because the user changed the password since the time the session was created or Facebook has changed the session for security reasons.

Can you go to your backend > Easydiscuss > Autoposting > Facebook > click revoke access > re-sign in with Facebook and see how it goes?
If the issues still persists, can you provide us with your facebook access so we can help you check on this? Please advise.