Messenger bubbles on macbook air

Hello martine bouillir,

It sounds like you have a question regarding notification settings for your Messenger app, right? Have you tried to enabled that notification and turn it off again?

Since it is a third party app, you may want to reach out to the app developer for more information:

How to contact an app developer - Apple Support

"Contact the developer of a macOS app

1. Open the App Store on your Mac.
2. Click the sign-in button  or your photo. 
3. A list of apps appears. Click the app that you want. If you can't find the app, use the search bar.
4. The app's product page appears. Click Support or App Support. If you don't see Support or App Support, make sure that you're signed in with the correct Apple ID."

Regards