How can I get Pages on my Mac Os Catalina 10.15.7.?

If you have installed Pages v5 or later via the Mac App Store, and you haven't changed your Apple ID, launch the Mac App Store, sign in with that same Apple ID, and press cmd+0 (zero) to open the Purchased panel.

If Pages appears there with an Update button or a blue outline cloud download symbol, click that. You will be presented with a dialog offering to install the last compatible version of Pages for macOS Catalina.

Otherwise, you will need to use Pages for iCloud.

In case: Pages User Guide for iCloud - Apple Support

If you've never downloaded it before you can't get an older compatible version. it requires macOS 12.3 or later.