Why do I have to update my iPhone to iOS 13.4 to use this app?
you must update to iOS 13.4 in order to download and use this application. what happened and why?
[Re-Titled by Moderator]
iPhone SE, iOS 13
Because the app developer (Meta) has chosen not to support any version of iOS older than 13.4. That means you need an iPhone 6S or newer.