The first thing to do when you get a problem like this is pass the error message to Google, and see if it comes up with anything.
In this case, it does: Google
This looks like a common problem with a common solution - this looks like it solves the problem: http://www.arhamzul.net/bagaimana-untuk-mengatasi-visual-studio-nuget-error/
] judging by the pictures, but I'm on a tablet at present and doing translations is PITA! Feed that to Google Translate, and it should show you the fix.