Hi Christian,
This issue might suggest only the file is open and not the project, as such Visual Studio will not have a reference to the various libraries required to understand specific classes. That said, in your screenshot Visual Studio hasn’t underlined things like Vector2 or transform for example.
I note that your screenshot also indicates that there is an update available for your installation. There was a significant issue in version 15.9.2, and VIsual Studio Tools for Unity 3.9.0.2, you can find details regarding this here;
If you have this version installed I would highly recommend updating it to avoid these issues.
Hope this helps 