Apple has noted that a number of users may experience an issue with iCloud setting being enabled after upgrading to iOS 10.3. For me, I had quite the opposite experience, as iOS 10.3 finally enabled some iCloud applications that I had never gotten working for a long time!
The problem is like this - every time I open Numbers (or the rest of the iWork suite like Pages and Keynote) on my iPhone 6, I'd get a message stating that iCloud was not enabled. So I'd dutifully head over to Settings and turn on iCloud for Numbers. On opening Numbers again, I'd be confronted with the same message, and toggling back to Settings would find the setting automatically disabled! Furthermore, iCould Drive is not present under Locations!
Repeating would only result in the same outcome, typifying the definition of insanity.
Worse, with Numbers on my MacBook, I'd be able to save to iCloud, but under iCloud Drive in iOS, I won't even see the Numbers folder!
Thankfully this is fixed with iOS 10.3. iCloud Settings are moved to a new and more prominent location, under what I'd describe as a "My profile" section, with Apple's apps given their own section:
Enabling iCloud for Numbers here finally works, and I hope it does for you too!