Why is Libre 2 diabetic app not working on iPhone 6 but works on iPhone 15 Plus?

This is never happened before. Is it labor two or iPhone? That’s having a problem with updates on older phones? I’ve done all of the things I should have turned the phone off, reloaded the app, if I delete his app, it will delete all of his data so I just remove it from the home screen and add it back on. I’ve turn phone off and on, I’ve checked all of these alarms and notifications And his app. They all are on, but it will not read his sensor. So I loaded his app on my phone and it works just fine. What’s going on?

[Re-Titled by Moderator]