Why can’t I pair my iPad Air 2 with my IPad Pro. What should I do?

What, specifically, are you hoping to achieve by pairing iPad’s with each-other over a Bluetooth connection?

Whilst it is technically possible to do so, for the majority of applications, Bluetooth pairing of an iPad and to another iPad or iPhone is entirely pointless. If your intention is to use Cellular Hotspot features of one iPad/iPhone with another, this link is optimally achieved using the native WiFi hotspot connection - this being a largely automated process. Hotspot connections over WiFi offer substantially greater throughput, range and reliability. As an alternative, a hotspot can be accessed via a direct cable connection (with an Adapter if needed).

How to set up a Personal Hotspot on your iPhone or iPad – Apple Support

If you still wish to mutually pair devices over Bluetooth, you first need to manually enable the hotspot feature on the device with a Cellular connection:

Settings > Mobile Data > Personal Hotspot > Allow Others to Join - set to ON

Next, Bluetooth for both devices must be placed in Discovery mode:

Settings > Bluetooth (accessing this page will automatically place the device in Discovery mode)

Whilst both devices in Discovery mode, you can mutually pair the devices.

Again, I must stress, unless you have an unusual “edge case”, mutual pairing over Bluetooth is largely pointless.