It sounds like installing the VS2017 update for that specific version didn’t also install the .NET Core 2.0 SDK. You can download that here.
To check which version of the SDK you’ve already got installed, run
dotnet --info
from the command line. Note that if there’s a global.json file in either your current working directory or any ancestor directory, that will override which version of the SDK is run. (That’s useful if you want to enforce a particular version for a project, for example.)
Judging by comments, some versions of VS2017 updates do install the .NET Core SDK. I suspect it may vary somewhat over time.