On Garmin Connect Web under Reports - Running - VO2 Max does not display any information. Why there is no data under running? I can see VO2 under cycling only. I use my Forerunner 910XT.
Did you go through the process for calibrating VO₂ Max described in the user manual for your watch?
Edit: I just noticed that I posted actually a link for a later model, but you will certainly find the same section in your manual too.