If you can install anything from the Mac App Store (free or not) then your are using the same Apple ID on Mojave too.
Open the Mac App Store, and if signed into it, click on your avatar/name in the lower left corner of it. This will take you to the Mojave version of the old Purchases tab. See if Keynote requires you to activate it?
If you have Keynote already installed as an older version, you can drag that application to a USB stick, unmount that, and drag Keynote to the Trash. Reboot, and then attempt to download Keynote 9.2.1 from the Mac App Store again.