Why does airSlate SignNow thinks that either my user-name or my password is invalid?

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.