Numbers doesn't get upgrade!

Apple will pop an informative dialog that a newer version of an <application> is in the Mac App Store when you are running any older version of that application. It does not care whether you can upgrade to it or not, nor is it implying any mandatory upgrade is required.

If any of your Pages, Numbers, or Keynote applications have either crashed, or. you force quit them, you won't be able to open documents until you press and hold the shift key while launching the application.

The versions of Pages, Numbers and Keynote available to download from the App Store have a minimum operating system requirement of macOS High Sierra 10.13.6.

Why can you no longer open the files you have on your El Capitan Mac what have you done differently, or were these files perhaps altered on another Mac running a newer version of Numbers and then imported back to your Mac, unfortunately there is no backwards compatibility.

Documents currently created in Pages, Numbers, and Keynote (v8.1, v6.1, v9.1), and the current iCloud counterparts, can be opened by Pages v5.6, Numbers v3.6, and Keynote 6.6 on El Capitan. What gets dicey is that the mentioned applications also consider certain obsolete Mac products as unsupported, and if for instance, one has El Capitan installed on a 2009 iMac, and Pages v{5.6, 5.6.1, 5.6.2} installed, the newer document won't open because of the unsupported hardware. The iWork '09 applications cannot open these new documents until they are exported in the iWork '09 document formats.