First get your project under source control if it isn’t already.
Then try reimport all to trigger a full recompile.
If that doesn’t work, try update all your packages, particularly the navmesh stuff and perhaps the Visual studio integration stuff.
This may help you with intellisense problems:
Also, try update the VSCode package inside of Unity: Window → Package Manager → Search for Visual Studio Code Editor → Press the Update button
Also, this: No suggestions in Vscode