I am geting this "You don't have permission to access /oauth/token on this server" error

I have a developer account for adobe sign but I cannot get my access token whenever I am trying to get it, it shows me "

You don't have permission to access /oauth/token

on this server

"

My user id and other account details are given below:

UserID : avijit.nath@cogitosoftware.in

Application ID: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Client Secret: xxxxxxxxxxxxxxxxxxxxxxxxxxxx

And I am trying to do this using the following url.

POST http://api.echosign.com/oauth/token?
code=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx&

client_id=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx&

client_secret=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx&

redirect_uri=https://salesmgmt.cogitocentral.com&

grant_type=authorization_code HTTP/1.1