how do I get my ipad 8th generation to backup from icloud?

I think you might be misinterpreting what you're seeing.

My guess is that the iPad has an older version of iPadOS (14.2) than the backup in your iCloud (which was made from a device running 14.2.1).

If this is a new iPad, you need to start at #1, otherwise start at #2:

1. Set the iPad up without a backup (temporarily).
2. Update it to iPadOS 14.2.1.
3. Then erase it and when prompted you can load your 14.2.1 backup from iCloud.

References:

Update iPadOS - Apple Support

Erase iPad - Apple Support

You must wait for an update to iPadOS. This should occur in the next few days.

sberman wrote:

You must wait for an update to iPadOS. This should occur in the next few days.

Doh! Quite right, iPadOS is lagging behind iOS by .1

Yes, only iPhone 12 models received the iOS 14.2.1.