How can I add my wife’s iPhone to AppleCare One?
How can I add my wife’s iPhone to my apple care one if it is not on my device list?
[Re-Titled by Moderator]
Original Title: Apple care one question
It sounds like your wife's iPhone is set up using her own Apple ID.
You can only add devices that are using the same Apple ID that you are to your AppleCare One plan.
Add or remove a device from your AppleCare One plan - Apple Support