Why does it doesn’t count steps if you play football with just your HRM pro on. It gives km distance so i think it should be possible.

HRM-Pro does count steps when you do not wear your watch, have the strap added to Garmin Connect, and allow the strap to sync with the phone over BT. 

From the FAQ:

HRM-Pro and Pro Plus: Top FAQs About the Activity Tracking Feature | Garmin Customer Support

The HRM-Pro® and HRM-Pro® Plus heart rate straps can act as an activity tracker by storing calories, intensity minutes and step data in addition to heart rate. When paired to the Garmin Connect® app, this data is synced and shared with your watch. This allows for cases when you might not be able to wear your watch, such as team sports and martial arts, but you still want to capture this data.

To ensure this data is captured make sure of the following:

1. Make sure the heart rate monitor (HRM) is paired with the Garmin Connect app.

2. Make sure you are wearing your HRM and be near your phone at the end of your activity. (Opening the Garmin Connect app on your phone will also cue a sync.)

   • The HRM-Pro/Pro Plus can store up to 18 hours of activity data before being overwritten.