Apple care

You can add AppleCare to a device within the first 60 days of purchase IF purchased directly from Apple or an Apple Authorized Reseller. Otherwise the date of ownership starts the date a device left Apple Inventory, if purchased not from an Authorized Seller.

So, when did you purchase your phone?

The most likely reason is you purchased your iPhone (new) more than 60 days ago.

There are other possible reasons, such as having purchased the iPhone (new) in a different country or region.