Package restore failed. Rolling back package changes

You can do the following steps:

1. VS Tools
2. Options
3. Nuget Package Manager
4. General
5. Clear All Nuget Cache(s).

OR

You need to change target .net core 1.1 to 2.0, and this is possible if you update your VS 2017 with the latest version(ex. VS 2017 15.3)