I just came to know that Invites do not work when SSO is enabled. Is their any way or any plugin so that Invites work even if SSO is enabled.
No, because invites must come from the parent site in that case. The parent site is responsible for user identity, not Discourse.
If you are using SSO, the “Invite” button still works in a topic. However, you can only use it to send a notification to a user that’s already registered has an account already in Discourse.