Can't erase iPhone for trade-in
Hi, I upgraded from an iPhone 8 to iPhone 13, trading in the 8. I have been following the instructions how to prepare iPhone for trade-in, but get stuck on one step. After delisting the iPhone 8 from my account, I am then asked to backup (already did weeks ago, thanks), then it asks for my apple ID password. That prompts a 6-digit verification code on the iPhone 13, "please enter this code on your device to sign in." But the iPhone 8 is stuck, nowhere to enter the 6-digit code.
Both are on iOS 15.3.1, both have WiFi access, SIM card removed from iPhone 8.
Any ideas?
iPhone 13 Pro, iOS 15