Http:// using postman, when i call this mattermost api i am getting "message": "Invalid or expired session, please login again.", error

Hello Harish

Did you log in first?
When you log in, the header of the response contains your token, which you need to provide in you future requests.
Then you set up a key-value in your request like this:
key: Authorization
value: Bearer r7xtos45fjrwxr4sn4bfocrnja
where you insert your own token instead.

Hopes this works.