Has it worked in the past or is this a new watch? Does the heart rate work? Everything updated?
The usual reasons are the band not being properly tight or positioned. Consider shutting off the phone, rebooting the watch, then powering on the phone. Make sure the band is tight, but not so tight it leaves a mark.
Try turning on sleep mode to see if that tracks sleep. If so then the watch is fine and the issues lies elsewhere.
