W/o a footpod, I think you must use the stock Treadmill activity for it to prompt for treadmill calibration
from the Tactix 7 manual:
Calibrating the Treadmill Distance To record more accurate distances for your treadmill runs, you can calibrate the treadmill distance after you run at least 1.5 km (1 mi.) on a treadmill. If you use different treadmills, you can manually calibrate the treadmill distance on each treadmill or after each run.
Start a treadmill activity (Starting an Activity, page 15).
Run on the treadmill until your watch records at least 1.5 km (1 mi.).
After you finish the activity, press STOP to stop the activity timer.
Select an option:
To calibrate the treadmill distance the first time, select Save. The device prompts you to complete the treadmill calibration.
To manually calibrate the treadmill distance after the first-time calibration, select Calibrate & Save > Yes.
Check the treadmill display for the distance traveled, and enter the distance on your watch
I have a Tactix 7 pro and the Treadmill activity came pre-configured out of the Box(!?).
When you want to add it, you can find it in the „Gym“ or „Indoor“ section (sorry, mine is not in english).
You can select between the classical treadmill and, if you have Zwift, you can also use it for those activities with virtual Run.
Please check it. Which Software version do you use?
Since the watch then uses the internal accelerometer with a non-optimal smoothing (my personal opinion). I also recommend a footpod to avoid dips in the pace when having a look at your wrist, or drinkink from a bottle.
it's one of the standard activities on the watch. maybe you deleted or moved it? easy to get it back if that's the case, but it's an amazing watch for treadmill activity as it has wrist based running power these days in addition to the usual running stuff. i have a stryd footpod and have alternated between the garmin native and the stryd. very different power numbers based on how they calculate, but the native garmin running dynamics is pretty great
