Can't sign in to existing Apple ID on iPhone 4

I found my iPhone 4, and for nostalgia purposes, wish to use it again (not as my primary device, but a secondary phone to my 8 Plus). However, the iPhone 4 only goes up to iOS 7.1.2 and I can't enter the Apple ID verification code required to sign in to my Apple ID. As you can see in the photo, I can't type in the verification code on my iPhone 4 because the keyboard doesn't pop up. There is an "OK" button which just dismisses the message. So the question: how can sign into my Apple ID on my iPhone 4?