Hi
Follow the instructions here to reply using dictation:
If the microphone / dictation button is currently disabled, then the following steps may help (check after each):
If you have not yet done so, update both your iPhone and your Apple Watch to the latest software, starting with your iPhone (be sure to make a backup first):
- Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support
- Update the software on your Apple Watch - Apple Support
On your iPhone:
- Go to: Settings > Siri & Search - check that Siri settings are enabled.
- Go to: Settings > General > Restrictions - check that the restriction for Siri & Dictation is not enabled.
- Go to: Settings > General > Keyboard - check that Enable Dictation is turned on.
Check that you are using a supported keyboard language for your Messages conversation:
Restart your Apple Watch and iPhone, turning both devices off together, then restarting your iPhone first: