Yellow triangle in References for general libraries

Just for documentation purpose **for new person with this issue** try this and you will rememberme :D

If you go to: Tools > NuGet Administrator > Configurations.

and you have "Allow nuget...." and "automatically check...." cheked.

The only thing than you have to do is click con the button **"Clear All NuGet Cache(s)"**

That's it, you don't have to edit manual thinks than can be dangerous, believe me, I use to need to done some of the steps than describe here a lot of time, and try more than 5 steps of the oficial microsoft documentation for that issue you could check it here: https://docs.microsoft.com/es-es/nuget/consume-packages/package-restore#restore-packages-automatically-using-visual-studio

But just cleaning the cache solve all the problems

 Option 1

Search for that dll in your project directory, if its exists then goto solution explorer -> references ->  browse and add that dll.

 

Option 2 

If that dll is not present in your directory then goto Tools > NuGet Package Manager > Package Manager Console command. 

 

Type  Install-Package yourDllName