I fixed this problem with clicking on regenerate project files in unity preferences-external tools
,I had the same Problem and today i could fix it.
In Unity-Preferences-External tools click on **regenerate project files.
Restart the visualStudio.
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.
I had the same problem when i downloaded visual studio checking unity
To fix it go to Edit>Preferences>External Tools>External Script Editor and select visual studio