If you use proxy, you will have to edit the Nuget.config file.
In Windows 7 and 10, this file is in the path:
C:\Users\YouUser\AppData\Roaming\NuGet.
Include the setting:
<config>
<add key = "http_proxy" value = "http://Youproxy:8080" />
<add key = "http_proxy.user" value = "YouProxyUser" />
</config>