NuGet v3

NuGet 3.x installs a NuGet.config file containing something like:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageRestore>
    <add key="enabled" value="True" />
    <add key="automatic" value="True" />
  </packageRestore>
  <packageSources>
    <add key="nuget.org" value="https://api.nuget.org/v3/index.json"
        protocolVersion="3" />
  </packageSources>
  <activePackageSource>
    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
  </activePackageSource>
  <disabledPackageSources />
</configuration>

scriptcs uses the NuGet.Core 2.x package and that ignores the nuget.org entry due to protocolVersion="3".

As a workaround, you can add an extra package source for nuget.org v2, e.g.

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageRestore>
    <add key="enabled" value="True" />
    <add key="automatic" value="True" />
  </packageRestore>
  <packageSources>
    <add key="nuget.org" value="https://api.nuget.org/v3/index.json"
        protocolVersion="3" />
    <add key="nuget.org v2" value="https://www.nuget.org/api/v2/" />
  </packageSources>
  <activePackageSource>
    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
  </activePackageSource>
  <disabledPackageSources />
</configuration>

You can also do this via the NuGet CLI:

nuget sources add -Name "nuget.org v2" -Source "https://www.nuget.org/api/v2/"

Alternatively you could add a file to your working directory named scriptcs_nuget.config, containing:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <add key="nuget.org v2" value="https://www.nuget.org/api/v2/" />
  </packageSources>
</configuration>