Error: Unable to load the service index for source

Did you just see this error when trying to create an extension?

error NU1301: Unable to load the service index for source https://dotnet.myget.or g/F/dotnet-core/api/v3/index.json  

This is either due to a proxy/internet issue, or you just need to log into Visual Studio.

Once you're logged in, the error will go away.