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

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 : REMOVED

Client Secret: XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

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

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

client_id=REMOVED

client_secret=REMOVED

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

grant_type=authorization_code HTTP/1.1

Please help me to resolve me this issues.

Moderator note:

You say you are a developer, so you should know better than to post private information in a public forum