Why doesn’t Scribble work on my Apple watch 4?

To use Scribble, the language for any given conversation needs to be a supported language for Scribble.

To change the language for a conversation, tap on Reply and then press firmly on the display.

More information:

Scribble on your Apple Watch - Apple Support

Hmmm it should work. Try my standard fixit list,

• Start with a forced reset, simply press and hold both buttons until the Apple logo appears, then release and let the watch reboot.
• Check and update software in your iPhone and Apple Watch (phone first). You should have iOS 12.2 and WatchOS 5.2
• If you didn’t need to update the watch software try unpairing then re-pair to reload the watch software. (Software update reloads anyway so this step would be redundant).

Where are you in the world? watchOS - Feature Availability - Apple