I am using VSC for Unity v2019.4. I have the package from package manager installed. Also I have installed dotnet. The thing is that when I open VSC (through Unity) it shows in the output: Attempted to update project that is not loaded: C:\workspaces\Unity\MyGame\Assembly-CSharp.csproj. am able to use the editor, but the Intellisense is not working. I also have tried to change the omnisharp version to latest and it is still not working.
You can try a few things in order to make Visual Studio Code works properly with Unity.
First of all make sure that the editor package is installed in your project via Package Manager. Latest version should be installed.
Just to be sure it's better to close the project, delete Library folder and all .csproj and .sln files.
Then you can open your project in Unity again, wait it to load and try to remember to open scripts not by double clicking, but instead use right click in Project window and select Open C# Project. This will load the scripts without any issues.
Try uninstalling then reinstalling vs code and dotnet latest version. Also make sure vs code is installed in a folder with permissions to edit. If not it will not save any files until you run as admin