Stamine

You will need a VO2max estimate. Without, it will not work.

support.garmin.com/.../

Exactly the problem is that I have like 10 routes with the stamina calculations without having a value in vo2max (in fact I don't have a potentiometer that is needed for cycling) and when I restore it it stopped working, thanks anyway

You will also get VO2max based on your daily HR. Thats why sometimes you will have a valid VO2max for the calculation. 

If you don't run or have a powermeter, that's the only way it worked before.

Thank you so much

https://support.garmin.com/en-US/?faq=lWqSVlq3w76z5WoihLy5f8

If you don't have a powermeter, your VO2 max estimate is coming either from running or walking activities (10mn+ at 70%+ max heart rate with GPS turned on). Although the watch can keep track of one VO2 Max for running and another one for cycling, it will use the running VO2 max for cycling-related calculations if the cycling VO2 max is not available

Thank you so much

Thank you all very much for the help, I went out for a 20-minute walk and it already marks the stamina in the mtb activity, I thought that the vo2max had to be unique to cycling, I hope I can help in the future