Cannot customize my Apple Watch faces

Ok. Make sure you are firmly pressing into the display of your Apple Watch. This should cause your Watch to vibrate, the face will shrink, and the customize button should appear. If not, make sure your Watch and iPhone are up to date (iOS 12.3.2 and watchOS 5.2.1) and try again. If up to date and nothing is happening, try turning your Watch off and back on again. If that still does not work, I’d continue using the Watch app on your iPhone or contacting Apple Support.

When you Force Press the display, are you able to see all the different face you can use? Or is it not doing anything? Of you are able to see other faces, is the problem just that you don’t see a customize button?

No, but you don’t need to do that until we understand the problem. When you Force Press the display, are you able to see all the different face you can use? Or is it not doing anything? Of you are able to see other faces, is the problem just that you don’t see a customize button? If you can take a screenshot by pressing the Side Button and Digital Crown at the same time, that’ll be helpful.

See if this helps....watchos

Try a reset by pressing in the crown and side button until you see the Apple logo, then release both.

As per the las Part of my suggestion, I’d continue using the Watch app on your iPhone or contacting Apple Support.