Based on your explanation, you are getting your build failed. For the following code,
var result=reservation.CanBeCancelledBy(new User{IsAdmin});
we can see the IsAdmin is the property of the User object, so when you define the new object, you need to assign a value to it. Try the following
var result=reservation.CanBeCancelledBy(new User{IsAdmin=true});
You may adjust the IsAdmin assignment based on your business requirement.
Hope this helps