can i download Pages to Mac OS 15?

I you ever downloaded and installed Pages prior to Catalina from the Mac App Store, then launch the Mac App Store, sign-in with the same Apple ID used to install Pages (v5 - *), and press cmd+0 (zero). This will open the Purchased page and on Catalina, you should see Pages shown with a blue cloud outline with a down-arrow. Click that and a dialog will appear offering to install the last compatible version (11.1) of Pages for macOS Catalina.

If you had not previously installed Pages from the Mac App Store, then you will not be able to get that last compatible version of Pages for Catalina, and will need to use Pages for iCloud, or Pages for iOS/iPadOS.

The current version of Pages requires macOS 11 or later.

There is no macOS 15 at this time. If you mean macOS 10.15 Catalina, then no, the current version of Pages is not compatible with it.

If you have at any point dowloaded Pages before, it should be in your Purchase History, and you may be able to get a compatible version from there.