Why can't the vortex smart trainer be used for an FTP test or to calculate Vo2 Max with an edge 520?

I have it connected to my edge 520 as an indoor trainer and can do power based workouts where I can maintain a cadence and it will adjust the resistance of the trainer to keep me in the required power zone.

When I go to the My Stats>FTP page the enable auto calculation is greyed out and it says no power meter paired.