参考网上大部分方法错误，根本不起作用。正确方法是

C:\Users\你的用户名\AppData\Roaming\NuGet找到NuGet.Config打开，看到类似下面信息（可能不一样）

<?xml version="1.0" encoding="utf-8"?>
<configuration><packageRestore><add key="enabled" value="True" /><add key="automatic" value="True" /></packageRestore><bindingRedirects><add key="skip" value="False" /></bindingRedirects><packageManagement><add key="format" value="0" /><add key="disabled" value="False" /></packageManagement><packageSources><add key="nuget-ch" value="https://nuget.cdn.azure.cn/v3/index.json" /><add key="Microsoft Visual Studio Offline Packages" value="C:\Program Files (x86)\Microsoft SDKs\NuGetPackages\" /></packageSources><activePackageSource><add key="nuget-ch" value="https://nuget.cdn.azure.cn/v3/index.json" /></activePackageSource>
</configuration>

找到关键词 https://api.nuget.org/v3/index.json

在关键词上下行加<disabledPackageSources>包裹起来，没有的话就把下面3行放对应位置

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

最后

<?xml version="1.0" encoding="utf-8"?>
<configuration><packageSources><disabledPackageSources><add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" /></disabledPackageSources><add key="huawei" value="https://repo.huaweicloud.com/repository/nuget/v3/index.json" /><add key="Microsoft Visual Studio Offline Packages" value="C:\Program Files (x86)\Microsoft SDKs\NuGetPackages\" /><add key="azure" value="https://nuget.cdn.azure.cn/v3/index.json" /><add key="tencent" value="https://mirrors.cloud.tencent.com/nuget/" /></packageSources><packageRestore><add key="enabled" value="True" /><add key="automatic" value="True" /></packageRestore><bindingRedirects><add key="skip" value="False" /></bindingRedirects><packageManagement><add key="format" value="0" /><add key="disabled" value="False" /></packageManagement>
</configuration>

注意：如果你项目里面有Nuget.config同样设置下 

【附录】

添加资源

华为国内镜像
https://repo.huaweicloud.com/repository/nuget/v3/index.json

微软国内镜像
https://nuget.cdn.azure.cn/v3/index.json

腾讯镜像

https://mirrors.cloud.tencent.com/nuget/
Microsoft Visual Studio Offline Packages
C:\Program Files (x86)\Microsoft SDKs\NuGetPackages\

国外nuget.org
https://api.nuget.org/v3/index.json

【参考文献】

[1] https://stackoverflow.com/questions/61380157/doing-a-local-nuget-package-installation-but-getting-the-error-unable-to-get-r