Galaxy watch doesn't give me an option to customize watch faces. Have tried it on my watch and also my phone. No customize button. I have an android, but not a Samsung. I'm not sure if that is why there is an issue, or if it could be something else?
You can read my article How to create a watchface
It uses a paid application but you can do unbeliaveble things then...
None of the watch faces that are customizable have the option to customize. On the phone app eaither. I have iphone and did not have this problem with my other galaxy watch. I wouldn't even care except my favorite face keeps changing to this red color.