Apple does not provide full installers for past versions of Pages. Pages v11.1, presently in the Mac App Store, requires that you have macOS 10.15.7 (Catalina) or later installed before you can download and install the current Pages application.
If you are using Pages '09 on High Sierra, none of the documents produced by Pages v5.0 through 11.1 are compatible with that retired version of Pages. Sign into the Mac App Store with your Apple ID, and then you should be able to access the Purchased tab while pressing the option key. This may show Pages with an Update button, and will either deny installing the current Pages or update the currently installed Pages to its last version for High Sierra. That would be v8.1.
You can always sign into iCloud and use the Pages application available there.