rcsdk in Send_SMS program have errors

In this code, I get the error for rcsdk.token.access_token.Length and rcsdk.Restapi().Account().Extension().PhoneNumber().Get(); like "The name 'rcsdk' doesn't exist in the current context"

static private async Task read_extension_phone_number()
        {
            if (rcsdk.token.access_token.Length > 0)
            {
                var resp = await rcsdk.Restapi().Account().Extension().PhoneNumber().Get();
                foreach (var record in resp.records)
                {
                    if (record.usageType == "DirectNumber")
                    {
                        foreach (var feature in record.features)
                        {
                            if (feature == "SmsSender")
                            {
                                send_sms(record.phoneNumber).Wait();
                                goto LoopEnd;
                            }
                        }
                    }
                }
            }

How can I fix it?

And also why this var string try to be a bool?

const string RINGCENTRAL_PRODUCTION = false;

Code from here https://developers.ringcentral.com/guide/messaging/quick-start#c#