You have to add it to the list of available apps on your watch. On the watch:
long press middle button on the left
go to "activities and apps"
scroll downwards until you see "+Add"
select stopwatch app to be added
Now you should see it when you press the "start" button (right top button)
This Simple StopWatch app looks very practical and user-friendly! I really like that it includes lap tracking with a sortable lap list — being able to toggle between index and duration (asc/desc) is a great feature for workouts and training sessions.
The key controls are clearly structured, especially the separate behaviors for when the stopwatch is running vs. stopped. The added Timer View with manual hour/minute/second adjustment makes it even more versatile.