Not sure Monodevelop is even supported as such anymore. Unity comes default with some flavor of Visual-something and it’s been that way for a few years.
Remember you can just use Notepad to edit files. Unity does not require you to have an IDE. The IDE gives Unity nothing. It is only for you.
In any case, perhaps some of the VS / VC stuff might be helpful, so I include it here:
This may help you with intellisense and possibly other Visual Studio integration problems:
Sometimes the fix is as simple as doing Assets → Open C# Project from Unity. Other times it requires more.
Other times it requires you also nuke the userprefs and .vsconfig and other crufty low-value high-hassle files that Visual Studio tends to slowly damage over time, then try the above trick.
Barring all that, move on to other ideas:
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