Getting prompted for verification code but where do I put it in?

Tack the numbers on the end of the password if you don’t get a pop up to enter the code.

That’s what happens on older os in general.

• Changing the Apple ID on a 1st generation iPad
• No input field for Apple ID Verification Code

To be clear, if your password were thirteen-odd-variations and your second factor (iMessage) were 330000, then enter thirteen-odd-variations330000 as your password instead of what you usually enter.

If you don’t see a password prompt at all, you’ll want to make sure you have a backup of your device and then sign out of iCloud / App Store / wherever you wish to sign in so that you get the account / password dialog.

• I’ve seen this happen when the certificate has expired and you need to sign out to release the old pairing from the keychain and then request a new session from iCloud

If you can’t ever get that dialog, you will want to restore the device from a computer or reset all content and settings - this erases everything, so be sure you have a backup or can afford to learn that the device has failed before doing the erase.