If your API credentials are correct, then you may have accidentally entered in an incorrect oAuth token, or your token might have expired. Please check that you are using a 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 errors - having a different content type other than what is specified in the example call. Make sure that it is Content-Type: application/x-www-form-urlencoded.
Why does signNow thinks that either my user-name or my password is invalid?