running Numbers on Mojave

The App Store needs to find that your AppleID has "purchased" Numbers. There are two ways to do this and they have already been covered in some detail in Old version (macOS Sierra) for Pages, Keynote and Numbers, iWork

Just needs a little adjustment for Yosemite rather than Sierra.

The way covered in detail is to install a pre-App Store version of Numbers and then the App Store will let you upgrade.

This is also described here and this is for Yosemite.

The second method is to install the current Numbers on another Mac (with macOS 10.15 or 11) using your App ID. Then the App Store will then let you go backwards to an older version on your Yosemite Mac.