Apple Watch

Are you trying to add new faces from the Watch itself, or from the Watch app on the iPhone? What happens when you try to add a new face? Change your Apple Watch face - Apple Support

Have you tried to change faces right from the Watch app on the phone? Select the face you want, and then tap on Set as current Watch face. That command will only appear when the face has been added to your collection.

Reset the watch. Hold the Digital Crown and power button together until you see the Apple logo and then release. The Watch will reboot.