My child's previous iphone not letting me sign him out, to factory reset; unknown error occurs everytime

I recently upgraded my son to an iPhone 16, he's signed in and has been using it ; I am now trying to factory reset his previous iphone 13 to sell to apple; but running into problems, I'm not able to sign him out of iCloud (running into a loop showing to disable stolen protection) but when trying to do so, it strangely shows: Sign out of iCloud: iPhone isn't a familiar location & Stolen Device Protection is on, need to turn off, etc; prompts to begin security delay.. but when his face ID is needed, after he attempts; an unknown error occurs everytime.. please help!