Access token validation failure Microsoft Graph API

Ideally, the resource should actually be

private const string resource = "https://graph.microsoft.com";

But you still need to select the scopes that you want to target in your application. The way you are doing it at the moment does seem to acquire/set the relevant scopes which is done for you by Graph Explorer.

I would suggest following this quick start tutorial on how to build a dot net core console app and you should be up and running in no time. It uses the MSAL library which works better than the ADAL library you are using in your scenario.

https://docs.microsoft.com/en-us/graph/tutorials/dotnet-core