Touch ID not working on iPad Air 2 after upgrading to iOS 14

That would be an update to iPadOS 14.0.1.

Perhaps you could better describe “not working”. What happens when you try to access and set-up TouchID is settings? What errors do you see?

Writing an effective Apple Support Commun… - Apple Community

A forced-restart of the iPad will often resolve an isolated issue that occurs after a system update:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

Further troubleshooting guidance for TouchID issues can be found here:

If Touch ID isn't working on your iPhone or iPad – Apple Support

The behaviour that you describe might suggest that the TouchID sensor and the OS are functioning normally - but your fingerprint may not be consistently read by the sensor.

A common cause of this problem can be due to grease or natural oils that are produced by your skin - or, if using barrier cream or moisturiser.

I have noted myself that difficulties can occur after using cleaning-gels during the current pandemic - after which I have had to sometimes had difficulties with my fingerprints being recognised, requiring that the fingerprint must be re-registered.

Climatic conditions can also cause issues, in particular when humidity is high.

Of current models of iPad, it is only the iPad Pro models that feature FaceID.