It looks like the IsScaffoldable extension method doesn't exist in the version of the T4Scaffolding.DLL that is installed with the NuGet package.
If found this work item which lead me to GitHub and I see this method exists. Since I'm not really using this attribute, I decided it's probably just simpler for me to remove the .IsScaffoldable() call from the T4 template instead of pulling down the source and compiling a new version of T4Scaffolding.