How to upgrade Numbers to version 12.1

The reason you can't upgrade to Numbers 12.1 is because you are not running Sonoma or Sequoia (macOS 14.x or 15.x) on your Mac. You must upgrade the version of macOS then you can download Numbers 12.1.

There is no "bug or error," Apple always shows you what the latest version of software is available. It does not mean you have to update. Even if you were able to download Numbers 12.1, and you cannot download it, it will not run on Big Sur. Just ignore the reminders.

The latest & newest version of any software is not necessarily the best. In an attempt to make something new, engineers make changes that don't always work for many users, whether it is software or a car or a product you buy at the grocery store. Change for changes sake is job security for them, usually nothing more.

I continue to use macOS Mojave, 10.14.6, & the iWork '09 apps (Pages 4.3 & Numbers 2.3), because they work best for me.

You have to upgrade your Mac to Sonoma or Sequoia to download the current version of Numbers. But there is no reason why you can't download the last compatible version as your screenshot says.