Try to figure out how to connect dev op by REST API without using PAT ID

I found info online. I also follow the steps. However, the sample code throw the exception. I need some help on it.

The following is the doc I followed.

https://docs.microsoft.com/en-us/azure/devops/integrate/get-started/authentication/oauth?toc=%2Fazure%2Fdevops%2Forganizations%2Ftoc.json&bc=%2Fazure%2Fdevops%2Forganizations%2Fbreadcrumb%2Ftoc.json&view=azure-devops#register-your-app