OAUTH setup - unauthorized_client ??

Good Morning

I am struggling to Configure Box.com access codes and thank anybody helping me.

My situation is the following:

I generated my exchange code.

Within a second, I request an access code:

POST https://api.box.com/oauth2/token 

grant_type=authorization_code
code=EXCHANGECODE
client_id=CLIENTID
client_secret=CLIENTSECRET

and it fails as I receive the following error message:

{ "error": "unauthorized_client", "error_description": "The grant type is unauthorized for this client_id" }