What do Yellow Warning Triangles mean on Dependencies in Visual Studio 2017?
Solution 1:
As trite as it sounds, try rebuilding then restarting VS - and get on with your day :)
Solution 2:
run dotnet restore before you do any complicated manipulation, you are going to be provided with a lot more info than in the error window or solution explorer.
You can run this command in the Package Manager Console:
Tools > NuGet Package Manager > Package Manager Console