Periodically, since Fall 2013, Apple released a version of Pages that is tied to the current operating system, and Pages/Numbers/Keynote v11.2 are now tied to Big Sur and the forthcoming Monterey. Apple does not provide user downloadable, past full installers for these applications, and if you do not already have Mojave/Catalina compatible versions installed, you won't be able too.
Your only hope for Mojave and Catalina, where you do have these applications installed, is to possibly get the last supported version of these applications for that operating system. Launch the Mac App Store, and then press cmd+0 (zero). This opens the Purchases panel, where you will see an update button on these applications. When you click it, you will get the dialog telling you that you need Big Sur. After you dismiss that dialog, you may/may not receive a second dialog offering to download the last supported version of the application for the current operating system, even if you have it already, in which case the Download button on that dialog will do nothing.
Otherwise, you can use Pages for iCloud, or an existing version of these iOS/iPadOS applications, or the v11.2 versions on iOS/iPadOS 14.8/15.1.
