tacx training minutes are recorded in Garmin app but not added as intensive training minutes

If you did not wear any Garmin device logging the heart rate, then you cannot get any Intensity Minutes from Tacx alone (even if you used a HRM strap).

And if you have worn a Garmin watch without starting an activity or HR broadcasting on it, then the HR sensor was in low power mode with slow sampling, and it might not have detected sufficiently elevated HR for any significant time. If you post a screenshot of the all-day HR graph from the given day, it could reveal it better.