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.