My VivoActive 4S records my VO2 max when I’m out using GPS. But dose not record when I do indoor exercise. What do I need to do to get it to record indoor exercise.
As the other answer - VO2max does not officially update on indoor runs. Indoor cycling does update VO2max for cycling (when the watch supports it and you have the right sensors)
There are NO obstacles technically for Garmin not be able to update VO2max on indoor runs - it is fully supported by FirstBeat (the software Garmin uses to calculate VO2max) - but it requires perfect "speed" measurements - which you can now get with Stryd, NPE Runn and other hardware (apart from Garmin own footpod which is still terrible)
But even if your VO2 max is not shown during indoor runs - calculations are still "updated" - if the watch support Race Speed Prediction - you will see your potential race speeds update after indoor runs.
But Garmin should really start doing more for the Indoor training space like Apple does with GymKit. I'm just waiting to check if my AW4 will update VO2max from GymKit connected Treadmill as the watch gets all the data direct from the TM during a run. Updates to follow...
Indoor you will only (so far) get VO2max updates when cycling - but then it is Cycling VO2max - which is different to the "general" VO2max which only updates while running outside with GPS and HR enabled.
No external sensors (yet) will make your running VO2max appear on a Garmin Watch - and it is due to Garmin not implementing those features from FirstBeat. Garmin COULD if they wanted to but so far they have not shown that initiative. As I keep saying - Garmin is terrible at support for Indoor activities.