The current versions (v14) of Keynote, Pages, and Numbers in the Mac App Store now require macOS Ventura (13.*) or later. Microsoft only supports new installations of Microsoft 365 or Office 2021 for Mac on macOS Ventura or later.
If you ever installed Apple's applications in previous versions of macOS since Oct 2013, then you may be able to install the last compatible versions (v12.1) for macOS Big Sur. That would involve launching the Mac App Store, signing in with the identical Apple ID used previously, and then pressing cmd+0 (zero) to open the Purchased panel.
If Pages, Numbers, or Keynote appear there with some means to download/install them, then click that and do not click Cancel on any subsequent dialog. Eventually, a dialog will appear offering to install the last compatible version for Big Sur. Do that.
Apple provides no download site for full, past versions of these applications.