我有一个类似的问题，试图连接到我的私人TFS服务器，而不是公共NuGet API服务器。由于某种原因，我在AD服务器和TFS服务器之间出现了问题，因此它总是返回401。NuGet配置文章展示了你可以像这样将你的AD用户名和密码添加到配置文件中:

  <packageSourceCredentials>
      <vstsfeed>
          <add key="Username" value="yourname@company.com" />
          <add key="Password" value="this is an encrypted password" >
          <!-- add key="ClearTextPassword" value="not recommended password" -->
      </vstsfeed>
  </packageSourceCredentials>

这不是一个非常理想的解决方案，更像是一个临时的解决方案，直到我可以找出AD服务器的问题，但这应该可以解决问题。

我在VS2010上使用的是旧版本的Nuget，默认为TLS 1.0 这里可以通过更改.Net框架使用的默认TLS版本来修复，该版本在注册表项中配置

reg add HKLM\SOFTWARE\Microsoft\.NETFramework\v4.0.30319 /v SystemDefaultTlsVersions /t REG_DWORD /d 1 /f /reg:64

reg add HKLM\SOFTWARE\Microsoft\.NETFramework\v4.0.30319 /v SystemDefaultTlsVersions /t REG_DWORD /d 1 /f /reg:32

FYI

6月15日，NuGet.org将永久移除对TLS 1.0和1.1的支持。请确保您的系统使用TLS 1.2。

您可以参考此链接了解TLS 1.2支持的信息

我在Visual Studio 2015上执行Install-Package Modernizr时遇到了类似的问题。我通过以下步骤解决了这个问题:

Download the package from its online source. Go to Tools/NuGet Package Manager/Package Manager Settings. Select Package Sources from the window. Add a new package source by clicking on the + sign. Enter a name and source location by clicking on ... (triple dot) sign. Make sure that only the package source that you've just added is checked. Uncheck all the other package sources. Go to Package Manager Console and type Install-Package Modernizr. Visual Studio 2015 installs the package automatically and creates Scripts and packages folders in your root folder.

我希望同样的解决方案在安装其他包时也能起作用。

I had the same error message while scaffolding Identity to my ASP.NET Core MVC project. Since my connection was not behind a proxy, removing/editing proxy configurations didn't make sense. And I didn't want to delete a file or uninstall PMC either. While looking around I realized a "Clear All Nuget Cache(s)" button on Tools --> Options --> NuGet Package Manager --> General. After pressing the button I had to wait for some time for the operation to complete. After that I tried to scaffold the Identity again but it didn't work. Then I decided to restart VS and voila :)

当我试图通过Jenkins(配置为服务，默认使用本地系统帐户)运行nuget.exe时，我偶然发现了这个问题。我已经编辑了C:\Windows\System32\config\systemprofile\AppData\Roaming\NuGet\NuGet。配置文件，如下所示:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
    <config>
        <add key="http_proxy" value="http://proxy_hostname_or_ip:3128" />
        <add key="https_proxy" value="http://proxy_hostname_or_ip:3128" />
    </config>

  <packageSources>
    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
  </packageSources>

</configuration> 

为了测试命令提示符，可以通过PSTools启动:

psexec -i -s CMD

和实际测试运行在新创建的cmd窗口(运行为本地系统):

path_to_nuget\nuget.exe restore "path_to_solution\theSolution.sln"

当我试图浏览NuGet包时，我得到了同样的错误，以解决以下相同的步骤:

1-转到%appdata%\NuGet\NuGet.config

2-验证该配置中提到的url

3-删除不需要的url

4-重新启动visual studio并检查