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?
It really depends on the watch face, not all watchfaces are customizable. Some are though (You can see the option on the wearables app when you open it ). The Frontier Watch Face does support this option if you want to test it out