Why isn't my iPad Retina iCloud Drive login verification code not showing up on my iPhone or Mac? I'm running an iPad retina, and trying to login to the iCloud Drive in the Settings App. 

It is logged into the App store, and iCloud sync for the Notes app. 

I also have an iPhone 6S and a Macbook Pro Retina. Normally the 2FA verification code (please enter this iCloud passcode) notifications show up on the other devices when doing iCloud related operations. (Turn on iCloud backups etc). 

When I try and login to iCloud Drive on my iPad, it asks me to login to iCloud with my password, and then pops up a 2FA verification code entry screen. But the code to enter doesn't come up on my other two devices. 

My question is: Why isn't my iPad Retina Cloud Drive login 2FA verification code not showing up on my iPhone or Mac?

Edit: 2FA is enabled on the other two devices. That's how the 2FA notifications mentioned above pop up on the other two devices for other iCloud operations.
It turns out you can trigger a 2FA verification code by going to:

iOS10: Settings > iCloud > Password and Security > Get Verification Code

iOS9: Settings > iCloud > [Your name at the top] > Password and Security > Get Verification Code

Now the challenge here is that the original iPad Retina was limited to iOS9. 

The other challenge is that the iPad retina running iOS 9 appears to have a 4-character 2FA - whereas my other devices are generating 6-character verification codes. I'll try and solve that somewhere else. 

There appears to be a trick to get the verification code sent by SMS.



Edit - it turns out it was asking me for the login passcode, not the iCloud verification passcode. Going to give the tick to the other answer to thank them for helping out.