How come I can't download certain apps on my IPad Air?

You may have restrictions turned on preventing you from downloading certain apps depending on their age rating. You can check restrictions in Settings > General> Restrictions. iOS: Understanding Restrictions (parental controls)

Then something is stuck. Go back to restrictions and turn restriction on. Create a passcode like it prompts you to. Go to the second section titled "Allow Content:" and change the App settings to allow all apps. Then go back to the app store and see if you get the button to download YouTube. Once the button comes up, you can then go back to restrictions and disable restrictions. This is the only thing in the iOS system that can stop you from downloading certain apps.

I knew it! Glad I could help 🙂

If you turned it off, the passcode goes away until you enable it again then it asks you to create a new one.

Even if your iPad is not compatible, it will give you the Free (or price button) or a cloud button, then when you try to download the app it will tell you about compatibility. The only thing that can make that free or cloud button disappear completely is restrictions.

If you still have a code in restrictions, see if you can guess it and then go to that setting like I said before and make sure it shows Allow All Apps.