Welcome Gudruupaksu,
It sounds like they built-in keyboard isn't working on your Apple Watch with the WhatsApp and Messenger apps.
At this time, these apps do not support using the built-in keyboard on the Apple Watch. This would be determined and supported by the app developer themselves. In order to reply to messages, you can use other methods of replying or type back on the paired iPhone's keyboard.
If you'd like to contact the app developer about this and possibly add that feature in the future, we'd recommend contacting them using: How to contact an app developer - Apple Support
Wishing you well, take care.