Lumo Run can be used on a treadmill, however the GPS function will display as a single point on your map because you appear stationary. Therefore your route, distance and pace will therefore not be as complete.
Your Lumo Run form metrics will remain unaffected by a change in your GPS function and therefore you will still be able to get information on your running form:
Cadence, Bounce, Braking, Drop and Rotation.
Please ensure your "auto-pause" within the settings is turned "off" while running on your treadmill, to ensure the "App" doesn't think you are stationary and therefore "pause".