Hello Carol-me,
Thanks for using Apple Support Communities. If we understand correctly, you’re having trouble with dictation within Facebook Messenger. We’re happy to help.
If you haven’t already, restart your iPhone and make sure you’re running the latest iOS version: Update your iPhone, iPad, or iPod touch
If that doesn’t resolve the issue, make sure your privacy settings are correct for microphone access: About privacy and Location Services in iOS and iPadOS
If this issue only happens in Messenger, try the steps outlined in this article for unexpected behavior with apps: If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open
Should the issue persist, you will want to reach out to Facebook for further assistance troubleshooting their app: How to contact an app developer
We hope that helps.
Take care!