I changed the form content type from form-data to x-www-form-urlencoded and tried it again. Now also the JWT Token error is persisting.
Note: It's programmatically generated (which by the way worked the first time we did it ), access token got generated and events were being called/invoked as well.
Then I came across the way to generate JWT token by using Adobe IO JWT tab.
Since the error in postman is telling us that the jwt token is incorrectly formatted. I thought I should generate the JWT token from there instead of creating it myself(which anyway is not generating the access token).
JWT token is not getting generated through Adobe IO console too.