Why does my phone say Important Display message?

Has the screen on your iPhone been replaced recently with a third-party manufactured screen? If YES, then that is the notification that you are getting on your screen.

You will see a notification on your Lock screen for the first 4 days that you use your device and in the Settings app for 15 days. After that period, you can find the notification by going to Settings  ➔ General  ➔ About

Axel F,

Mhizfahad39 Said:

"Why does my phone say Important Display message?"

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Did you Get a New Screen?:

If you are getting the notification, then the issue is that is an ungenuine display. So, did you have this screen replaced? If so, then ask the person who replaced it to replace it once more, but this time with a genuine screen. Elsewise, you'll continue getting the notification. They should have known better  Learn More here: About Genuine iPhone Displays - Apple Support