If you are having trouble deleting a message on Messenger that won't delete on your iPhone, there are a few troubleshooting steps you can try.
1. Force quit the Messenger app by double-clicking the home button and swiping up on the Messenger app to close it. Then, reopen the app and try deleting the message again.
2. Clear the cache and data for Messenger by going to Settings > General > iPhone Storage, scrolling down to Messenger, and tapping on "Offload App" or "Delete App." Then, reinstall the Messenger app from the App Store and try deleting the message again.
3. If the message still won't delete, check your internet connection to make sure you have a stable connection. You can also try signing out of Messenger and signing back in again, or restarting your iPhone.
If none of these steps work, you may need to contact Messenger's support team for further assistance.