If you want to see any unsent messages on Messenger, follow these instructions:
- First of all, Open the Messenger app.
- Then, Tap More (three dots) which is present at the bottom right of the screen, then choose the options of Settings that appears in the menu.
- After that, choose the options of Chat Settings which is available at the top of the page, and check that Send Read Receipts options must be turned off.
Read more here:- https://worldzo.net/how-to-see-unsent-messages-on-messenger/