You may have accidentally entered an incorrect oAuth token, or your token might have expired. Please check that you are using the oAuth token and not encoded-client-credentials. Also, make sure that for the sandbox environment, you are using api-eval endpoint, and that the username&password have login credentials for the test-web-application account, not for production. If everything looks correct, try refreshing your token using 'http://app-eval.signnow.com/oauth2/token' and resubmit your API-call again. Another possible source of error - having a different content type other than what is specified in the example call. Make sure that it's Content-Type: application/x-www-form-urlencoded