Why does my Certificate service says expired

You got here from a warranty webpage, but are asking about certificates.

I’ll assume you’re looking at iPhone Settings > About > Coverage here, and not certificates.

An iPhone X is almost certainly out of warranty, and may or may not still have AppleCare+ coverage.

If you’re asking about certificates, you’ll need to provide a little more detail.

If you’re getting a certificate-expired warning, you might have a subscribed calendar.

You can check for and remove that: Delete spam calendars and events on iPhone - Apple Support