I have a developer Box account where I created a custom app (client credential flow), I managed to get an access token using postman and make some requests using it. However when I'm trying to make a request from the Salesforce I'm getting the next error: 
Unable to fetch the OAuth token. Error: unauthorized_client. Error description: The "box_subject_type" value is unauthorized for this client_id.

Can anyone help or explain what I'm doing wrong.