Getting the below error in my project as well. I tried the solution in the locked thread below, but that did not work ( or at least was incomplete ).
If you check the Extensions -> C# -> Changelog you will see the following:
- For Mono-based development (e.g. Unity) that requires full .NET framework, you need to set
"omnisharp.useGlobalMono": "always". The current value of “auto” will remain “never” until Mono upgrades their bundled MSBuild version.
To correct this you need to go Code -> Preferences -> Settings
Search for “omnisharp.useGlobalMono” and set the value to always, this will remove the error.
FYI
Unity 2020.2.6f1
VSCode 1.53.2
OS: macOS 10.14
[fail]: OmniSharp.MSBuild.ProjectLoader
The reference assemblies for .NETFramework,Version=v4.7.1 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks
[warn]: OmniSharp.MSBuild.ProjectManager
Failed to load project file '/Users/sryan/code/Test Project/Assembly-CSharp.csproj'.
/Users/sryan/code/Test Project/Assembly-CSharp.csproj
/Users/sryan/.vscode/extensions/ms-dotnettools.csharp-1.23.9/.omnisharp/1.37.6/omnisharp/.msbuild/Current/Bin/Microsoft.Common.CurrentVersion.targets(1180,5): Error: The reference assemblies for .NETFramework,Version=v4.7.1 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks```