The last version of Keynote compatible with macOS 10.15.7 was Keynote v11.1. The current Keynote v14 in the Mac App Store requires macOS Ventura (13.*) or later.
If you personally and previously installed Keynote from the Mac App Store in the past, then launch the Mac App Store, and sign in with the same Apple ID you used when installing Keynote. Press cmd+0 (zero) to open the Purchased panel. You should see an entry for Keynote and something obvious about downloading it (might be a blue cloud icon with a down arrow). Click that and do not click Cancel on any subsequent dialog. The last dialog will offer to install the last compatible version of Keynote for macOS Catalina.
Alternatively, if Apple or a third-party reseller pre-installed Keynote on your Mac when new, then you won't be able to download the last compatible version for Catalina as you are not the original Apple ID holder. Apple provides no other means to obtain older full installations of Pages, Numbers, or Keynote.