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.