Greetings CherryYogurt,
Per the App Store, Procreate requires iPadOS 14.4 or later and will not be available to purchase on a device running an earlier version of iPadOS or iOS. While your iPad Air 2 can run iPadOS 14.4 or later, if it is not currently updated to there or later you won’t be able to purchase the app.
You will still need to connect your iPad Pro to a Wi-Fi network or personal hotspot to download and install the app on your iPad Pro. If your cellular plan supports Personal Hotspot, enabling that on your iPad Air 2, and then connecting your iPad Pro may be a way to go to get the app installed. Here’s how to set it up on both devices - How to set up a Personal Hotspot on your iPhone or iPad
Thanks.