It sounds like you might be missing the Visual Studio plugin for Unity.
Do this ... in Visual Studio go to View in the main menu and see if you can see the Unity Project View option ... it should be near the top and has a hot key combo of (Shift + Alt + E) ... if you do see this then you do have the Unity tool installed ... if not you do not ... so install it ...
How to install it?
Using Visual Studio Installer modify the install for your Visual Studio and scroll down to the Gaming section ... select Game Development with Unity. See the attachment for an example.![Image showing the Game Development With Unity plugin for Visual Studio][1] [1]: /storage/temp/150157-capture.jpg
Do this ... in Visual Studio go to View in the main menu and see if you can see the Unity Project View option ... it should be near the top and has a hot key combo of (Shift + Alt + E) ... if you do see this then you do have the Unity tool installed ... if not you do not ... so install it ...
How to install it?
Using Visual Studio Installer modify the install for your Visual Studio and scroll down to the Gaming section ... select Game Development with Unity. See the attachment for an example.![Image showing the Game Development With Unity plugin for Visual Studio][1] [1]: /storage/temp/150157-capture.jpg