I also had this issue. “Add Controller>API Controller with actions, using Entity Framework” would give the “Package Restore Failed” error.
As Anish stated, it seems to be due to package versions being mis-aligned. I was able to resolve this issue using “Manage NUGET Packages for Solution”, then performing an “Update All”. This set my AspNetCore version to 2.1.5 and resolved my “Package Restore Failed” error, but then led to another error, “NETCore version 2.1.5 not found”. Apparently the scaffolding code generator needs the AspNetCore and the NETCore versions to be in sync, so I manually downloaded and installed the NETCore version 2.1.5 from Microsoft Downloads. This worked, and I was finally able to generate Controllers.