You are in video mode and not photo mode when your drawing the box?
I use spotlight mode all the time with my Air 2 and you just go to the video mode and drag a box around the target this activates spotlight mode (the eye symbol at the bottom of the screen) this mode locks your target in centre frame and allows you to manually fly around objects super smooth while the drone does all the work keeping the subject framed.
Maybe this will help you:
The 2nd video I posted is all about spotlight mode in the above 