API issue - Access denied (Get message and Post message)

I followed the documentation here and setup the API channel: https://support.snatchbot.me/v1.0/reference#post-message

When I make calls to the API endpoint(with the app key and secret) obtained when I added the channel, I'm getting the error below:

{"error":"Access denied","status":false}

I tried both the Get Message and Post Message(with header Content-Type: application/json, user_id as query param and message in the json body).

I used the user_id obtained from the log of a JSON API interaction within the same conversation.

Am I missing something here! Any help is much appreciated.