Vo 2 max cycling

4898640 said:

What can be the reason? Any ideas?

The required prerequisites are listed at What Is VO2 Max Estimate and How Does It Work? | Garmin Customer Support, so check them out, and verify whether they were really all met:

Activities recorded from these apps do not generate a VO2 max reading by themselves. The activities are post-processed using a Garmin device and Garmin Connect to generate a reading. For your VO2 max to update from a supported fitness app, you must meet these requirements:

• The app must be linked to your Garmin Connect account. Tacx apps are linked by default.
• Your VO2 max capable Garmin device must be listed in your Garmin Connect account. This allows the activity recorded from the other app to sync with the device to allow post-processing.
• Your VO2 max capable Garmin device must have an activity of the correct activity type saved on it that has produced a VO2 max estimate. It is recommended to have a recently recorded activity to provide the most accurate information.
• Physio TrueUp must be enabled on your Garmin device (see What is Physio TrueUp?).
• Qualifying activities require heart rate data. Cycling activities requires heart rate and power data.

If these requirements are met, Garmin Connect will sync the activity data to your Garmin device. The device will then process this data and, if conditions are met, an updated VO2 max reading will be generated. If you are using an Edge® fitness device, you must sync your device twice for an updated VO2 max to display in Garmin Connect.