OmniSharp.MSBuild.ProjectManager Attempted to update project that is not loaded:

I found another solution since none of the other solutions were working for me and i was searching for hours.(07/08/2021):

I got the same error with .NET, .NET Core or .NET Framework, here are some screenshots of the errors:

In the error output, it says to install the Developer Pack for this framework version. You can find the version in the .csproj file:

By installing the .NET Framework of this version on the dotnet microsoft site: .NET Framework download list i fixed the error after reloading VSCode and got my Intellisense working for Unity.

TLDR: My fix was too install .NET Framework version 4.7.1

Solved (cross checked on Ubuntu 16.04 and CentOS 7):

In the VS Code menu, go into File / Preferences / Settings.

In the "Search settings" window, type omnisharp.path

Click on "Edit in settings.json" link.

Add (or edit) like this: "omnisharp.path": "latest"

(from: https://github.com/OmniSharp/omnisharp-vscode/issues/2965)

I solved this issue by doing the following on VSCode:

1. Go to settings. (Code -> Preferences -> Settings on Mac)
2. Search "omnisharp".
3. Find the section "Omnisharp: Use Global Mono," and set it to "always."
4. Find the section "Omnisharp: Path," and click "Edit in settings.json." Change "omnisharp.path" as "omnisharp.path": latest".
5. Reload the window.