I’m trying to block someone on Messenger but as soon as I close the app and get back in the person

If you go to the text messages with you and that person, go to the settings and press " block this person" Once you do that it will tell you that the person has been blocked. If it does not tell you that then you will need to go to the messenger settings a look for the button that says block a person, once you pick the person you want to block, it will block the person.

I hope this helps you, have a great day!