Hello. My friend actually had the same problem when she first bought her Samsung Galaxy S8. Luckily she was able to solve it with these steps. Here are the steps:
1. Launch your Settings.
2. Head over to Lock Screen and Security.
3. Tap the 'Always ON Display option.
4. Use the option on the left named 'Clock Styles.
After following those four steps, you will then be able to change your clock style.