auth code flow - OAuth2::Error: invalid_client: Client authentication is required

Hi, I am using the standard ruby [ringcentral sdk][1] Here is how I am trying to authenticate a user using the auth code flow. Step 1 @client ||= RingCentralSdk::REST::Client.new do |config| config.server_url = RingCentralSdk::RC_SERVER_SANDBOX config.app_key = "client_id" config.app_secret = "client_secret" config.redirect_url= "redirect_url" end Step 2 I provide my consent using a browser with url `@client.authorize_url` Step 3. I copy the code returned by the api and try to exchange it with a token like so token = @client.authorize_code(code) No matter what I do, I always get this error { "error" : "invalid_client", "errors" : [ { "errorCode" : "OAU-123", "message" : "Client authentication is required" } ], "error_description" : "Client authentication is required" } Here are my Ringcentral app settings **App Type:** - REST API App (most common) **Auth:** - 3-legged OAuth flow authorization code, - Server-side web app **App permissions:** - VoIP Calling **Who will be authorized to access your app?** - This app is public and will be callable via any RingCentral customer. **What brand partners are available?** - Make available to all current and future RingCentral carrier and brand partners(recommended) I have even done several combinations of these settings to no luck [1]: https://github.com/ringcentral/ringcentral-ruby