Download 14.8.1 says error (null)

Apple recently released iPadOS 15.1 and earlier versions should not be available any longer. Your iPad should be compatible with the latest iPadOS release and what it would upgrade to if it is running iPadOS 14.x now.

However, if it is having issues upgrading over Wi-Fi, then see the following support article for assistance:

• If your iPhone, iPad, or iPod touch won't update - Apple Support

Ref: About iPadOS 15 Updates - Apple Support

Where are you attempting to download iPadOS 14.8.1 for your iPad? Which exact model is your iPad? Are you trying to update/upgrade this iPad over the air (Wi-Fi) or by connecting it directly to a computer?

Apple released both iPadOS 15.1 and iPad OS 14.8.1 (and the same for iOS), following their new option of allowing users to remain on 14 and receive security updates only, or choose to upgrade to 15 for new features and security upgrades. Specifically under 14, version 14.8.1 is available, but 14.8 and earlier are no longer available.