Using ICollection in Rest API Controllers to get specific Data

Question

So I'm building my first Rest API and I have those 3 Model classes.

And here is my DbContext

Now I want to do this: I want to create a GET Method in the DeskController that shows me every Desk where bookings = null. I tried to construct it myself but I have problems with the error message "Unsupported Media Type" when I try to reach the route.