As of May 2022, OmniSharp doesn’t include MSBuild anymore. Therefore you will notice that Visual Studio Code doesn’t respond to your keystrokes if you have newly installed the Unity and don’t have MSBuild already installed.

The solution is installing MSBuild manually. Start Visual Studio Installer and choose “Modify” on your latest version IDE or the one you use with Unity. Under the Individual Components tab, choose MSBuild and click the “Install” button. Restart Visual Studio Code. Try again to type if your OmniSharp is running.

For further information on MSBuild: