.NET Core中的build命令出错（未知此类主机）

我正在.NET Core中制作一个基本的控制台应用程序，当我尝试运行命令“dotnet build-c release-r win10-x64”时，我得到以下结果：

Microsoft (R) Build Engine version 16.5.0+d4cbfca49 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

  Retrying 'FindPackagesByIdAsyncCore' for source 'http://packagesource/FindPackagesById()?id='Microsoft.NETCore.App.Runtime.win-x64'&semVerLevel=2.0.0'.
  No such host is known.
  Retrying 'FindPackagesByIdAsyncCore' for source 'http://packagesource/FindPackagesById()?id='Microsoft.WindowsDesktop.App.Runtime.win-x64'&semVerLevel=2.0.0'.
  No such host is known.
  Retrying 'FindPackagesByIdAsyncCore' for source 'http://packagesource/FindPackagesById()?id='Microsoft.AspNetCore.App.Runtime.win-x64'&semVerLevel=2.0.0'.
  No such host is known.
  Retrying 'FindPackagesByIdAsyncCore' for source 'http://packagesource/FindPackagesById()?id='Microsoft.NETCore.App.Runtime.win-x64'&semVerLevel=2.0.0'.
  No such host is known.
  Retrying 'FindPackagesByIdAsyncCore' for source 'http://packagesource/FindPackagesById()?id='Microsoft.WindowsDesktop.App.Runtime.win-x64'&semVerLevel=2.0.0'.
  No such host is known.
  Retrying 'FindPackagesByIdAsyncCore' for source 'http://packagesource/FindPackagesById()?id='Microsoft.AspNetCore.App.Runtime.win-x64'&semVerLevel=2.0.0'.
  No such host is known.
C:\Program Files\dotnet\sdk\3.1.201\NuGet.targets(124,5): error : Failed to retrieve information about 'Microsoft.NETCore.App.Runtime.win-x64' from remote source 'http://packagesourc
e/FindPackagesById()?id='Microsoft.NETCore.App.Runtime.win-x64'&semVerLevel=2.0.0'. [C:\Users\steve\PlatziCSharp\PlatziCSharp.csproj]
C:\Program Files\dotnet\sdk\3.1.201\NuGet.targets(124,5): error :   No such host is known. [C:\Users\steve\PlatziCSharp\PlatziCSharp.csproj]

Build FAILED.

C:\Program Files\dotnet\sdk\3.1.201\NuGet.targets(124,5): error : Failed to retrieve information about 'Microsoft.NETCore.App.Runtime.win-x64' from remote source 'http://packagesourc
e/FindPackagesById()?id='Microsoft.NETCore.App.Runtime.win-x64'&semVerLevel=2.0.0'. [C:\Users\steve\PlatziCSharp\PlatziCSharp.csproj]
C:\Program Files\dotnet\sdk\3.1.201\NuGet.targets(124,5): error :   No such host is known. [C:\Users\steve\PlatziCSharp\PlatziCSharp.csproj]
    0 Warning(s)
    1 Error(s)

Time Elapsed 00:00:08.30## Heading ##

似乎它缺少了一些依赖项，我对这一点非常陌生，在网上找不到任何答案 任何人都可以帮助我如何安装我丢失的东西？ 在过去，我试图做另一个.NET核心项目，但同样的错误出现了，我放弃了它，这是如此令人沮丧

提前谢谢

在工具选项…NuGet软件包管理器软件包源， 按源添加包源

在工具NuGet软件包管理器软件包管理器控制台上输入dotnet restore命令

关于工具>选项…>NuGet软件包管理器>软件包源，添加 一个源一个源的包

重新启动Visual studio并重新加载解决方案/项目

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运气好吗？我遇到了同样的问题，这是：