Currently Comments field let you add message even to the closed ticket but we have a requirement that it should let add a message only when a ticket is open.I'm not sure how should I go and do it any leads will help a lot,.
This would be done via ACLs. No scripting necessary - a condition builder such as "Active | is | true" is likely granting write access to one or more fields while the record is active and another ACL is granting access to comments after that. Ensure your access control lists prevent writing after the record is closed.
