2. Make sure your Android device is running 4.4+.
Go to Settings on your phone > General > About phone > Common > Software info > Android version
3. If your Android device is running 4.4.2/4.4.3/4.4.4, when you get a notification pop-up includes more than 2 messages, you can't reply to this meesage. Please read those messages on your device first.
e.g. There are 2 messages in the first pop-up, and you can't reply to this message (This issue doesn't happen to devices running Android 5.0+):
Detailed guide on how to Reply WhatsApp Messages.