Because you have no devices that have Find my iPhone enabled that are logged in to the Apple ID that the Find my iPhone app is logged in to, and none in your family sharing plan (if you have one) that are logged in to the associated Apple ID.
Because you have no devices that have Find my iPhone enabled that are logged in to the Apple ID that the Find my iPhone app is logged in to, and none in your family sharing plan (if you have one) that are logged in to the associated Apple ID.
Why does “no devices registered” come up when I log in to find my IPhone?