Hello nene80,
Thank you for posting in the Apple Support Communities. We'd like to help resolve this notification issue with the Messages app.
For this issue, we recommend following steps 1-3 in the Apple Support article below. Closing then reopening the Messages app, restarting your iPhone, and checking for updates can help resolve many unique issues.
If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open
If you see a new iOS 15 update available, be sure to back up your data to iCloud or to a computer, first.
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
If this concern continues after taking the steps above, our resolution is to contact Apple Support directly. Depending on your region, you may be able to reach them here:
Get Support
We hope this information is useful. Kind regards.