How old is your iPad, is it on ios 12 or earlier?
Some of the old iPads don't have the feature to enter the verification code and instead you enter the code in the same box as your password, directly after it.
If you can, update your iPad if it's able to.
From: https://support.apple.com/en-au/HT204915
What if I use two-factor authentication on a device running older software?
If you use two-factor authentication with devices running older OS versions—like an Apple TV (2nd or 3rd generation)—you might be asked to add your six-digit verification code to the end of your password when signing in. Get your verification code from a trusted device running iOS 9 and later or OS X El Capitan and later, or have it sent to your trusted phone number. Then type your password followed by the six-digit verification code directly into the password field.