Here is the solution:

1. Open Visual Studio and go to Tools > Extensions and Updates
2. Uninstall "NuGet Package Manager for Visual Studio 2015"
3. Restart Visual Studio 2015
4. Open Visual Studio and to Tools > Extensions and Updates again
5. Install "NuGet Package Manager for Visual Studio 2015"
6. Restart Visual Studio 2015

I got a similar error message when trying to create an MVC Web App on VS2015. Found this solution and it worked for me.

1. Go to Tools>Extensions and Updates
2. Search the NuGet Package Manager for Visual Studio 2015
3. Install NuGet Package Manager
4. Restart VS2015

Hope it helps.

On VS2017 go to "Tools-> Extension and Updates" then find and Check "Xamarin for Visual Studio", it was disable, i enabled it, and restarted. Worked for me.

Refer: https://developercommunity.visualstudio.com/content/problem/83460/cannot-create-a-new-xamarin-project-using-vs2017.html