Authorize API with Web Application Failed

If it's a sandbox app, you can simply create a new app instead to enable password flow.

By the way, public app doesn't support password flow.

If you have difficulty configuring it, please contact the developer support team or submit a support ticket.

This a common issue user face. Similar type of question raised here.

More precisely discussed here.

If you're using an OAuth 2.0 request using the OAuth 2.0 password grant (grant_type=password), then your application you created should in the RingCentral Developer Portal should be Password flow .

Please note, application created with different platform type will have different grant_type.

For example, application created for platform type = browser based will have grant_type=Authorization Code | Implicit | Refresh Access Token where as application for platform type = Desktop/Windows will have grant_type= Authorization Code | Password flow |Refresh Access Token

So in order to use (grant_type=password) you need to have application created with platform type that support Password flow

Here is wonderful way it has answered with animated Gif image that will easily help you to understand the proccess: https://stackoverflow.com/questions/47692828/ringcentral-auth-token-failed-in-curl-call-unauthorized-for-this-grant-type/47745436#47745436