My iphone 12 pro no sound of messenger notification. Please help me out. Thank you

Make sure you have the latest app updates. Check your settings under notifications as well as check your settings in the app. If the issue persists reach out to Facebook for assistance.

Hello,

Are really referring to "messenger." If so, Messenger is a Facebook sub-application and I don't believe that it plays a sound. See this: How to contact an app developer

Instead, if you mean the iOS app Messages, see this for the standard steps for no sound (may apply to Messenger as well):

1) Is the side silent mute button on? Does any red show? Slide it back and fourth.

2) How about the volume buttons? Is the volume very low? Move the buttons.

3) Is Do Not Disturb on? Go to Settings > Focus > Do Not Disturb. Is it turned on? Check the times.

4) Go to Settings > Sounds & Haptics. Check the Ringer and Alerts slider.  Try changing the sound pattern for the type of alert.

5) Go to Settings > Notifications. Is the app listed?

6) Do other sounds work? What about phone calls? Music? 

7) Volume summary:  Adjust the volume on iPhone

8) Do you have an Apple watch? Many of the notifications will go to the watch instead of the phone.

Note: you can send text messages to yourself for testing purposes.

Good luck.