Did you install it on your watch ?
If No:
Can you try to search for the watch face directly by using the Google Play Store on your watch? Just search the app by its name 🙂
Or, you can go to the Google Play Store on your watch, in the “installed on my phone” category, and install the associated watch app.
If Yes:
Hold your finger on the screen until the watch face selector is displayed. Then scroll among the installed watch faces to find it. If you do not find it, go to the last right item “+ add watch face” of the list to display all watch faces installed.
And then, install the companion app on mobile for a full experience.