Garmin Instinct (1st gen) does not support the VO₂max feature. You need either Instinct 2 or Instinct 3 for that, and if you own one of those, then you find the data either on the VO₂max widget on the watch, and similarly on the Performance Stats » VO₂Max screen of the app, or online at https://connect.garmin.com/modern/report/21/all/last_four_weeks. It is also available on the Training Status screens.
