Apple Watch series 4

I would suggest you reset your watch. Press the crown and side button in untill the Apple logo appears. Also Check your Watch is updated to 5.3 and your iphone is running ios 12.4. Finally still not able to add faces Unpair your Watch and repair it.

How to add additional versions of Apple Watch faces via Apple Watch

1. Press firmly on the watch face to enter switching mode.
2. Swipe all the way to the left and tap + New button.
3. Turn the Digital Crown to select the clock face you want to add.
4. Tap the watch face to confirm.
5. Customize the new watch face version.