You might need to install Visual code or Visual Studio separately nowadays, as this version of Unity is extremely ancient.
Remember: you don’t NEED Visual Studio or Code to work in Unity. You can use any editor you like (even Notepad) to edit your files.
You should consider coming ahead to a more recent LTS version of Unity if you can.
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