Hi Hernan,

First of all, next time please make you question clear so we can help better. Also don't post sensitive data in your question (it is not safe to reveal the client id and secret even you just encoded with based64).

Back to your question. I think you specified a wrong platform type to use with password flow authentication. For password flow login, you have to create an app with the platform type set to "Server-only (No UI). So please create a new app with correct platform type and try again.

Also, is there a reason why you are not using RingCentral SDKs? If not, please check out RingCentral official SDKs.

If you are a beginner, these quick starts would also help you quickly get onboard to develop your app.

This type of question has been asked multiple type in this forum. Most of the case the issue arise with the wrong platform selection for the application. The public and private app will have different grant_type.

Things to check id your Client id & secret. Make sure you written it correctly and converted into base64. Make sure you use correct extention.

Make sure the application you created supports Password flow since your (grant_type=password) .

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

for more info, 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