You can only deactivate Messenger if you previously deactivated your Facebook account.
To deactivate Messenger after you’ve deactivated your Facebook account:
- Open Messenger
- Tap your profile picture in the top left corner > Privacy & Terms > Deactivate Messenger
- Enter your password and tap Continue
- Tap Deactivate
To reactivate Messenger, log in with your username and password.
If you deactivate both Facebook and Messenger and then reactivate Messenger, your Facebook account will also be reactivated automatically.