无法加载项目：The project file cannot be opened by the project system

The project file cannot be opened by the project system, because it is missing some critical imports or the referenced SDK cannot be found.
Detailed Information:
Unable to locate the .NET Core SDK. Check that it is installed and that the version specified in global.json (if any) matches the installed version.
中文会报这个：
无法找到 .NET Core SDK。请检查确保已安装此项且 global.json 中指定的版本(如有)与所安装的版本相匹配。

找到根目录下的 global.json 文件，
查看 SDK要求的 .Net Core 版本，如下图要求 3.1.100

{
  "sdk": {
    "version": "3.1.100",
    "allowPrerelease": false,
    "rollForward": "latestMajor"
  }
}

global.json 概述 参考：https://docs.microsoft.com/zh-cn/dotnet/core/tools/global-json?tabs=netcore3x

到这里下载对应的版本、并安装：https://dotnet.microsoft.com/download/dotnet-core

如果只是查看代码，而不需要编译执行，可以把global给去掉，就可以直接打开了。