The Mac App Store only has the latest version of applications for a new purchase. In the case of Keynote version 7.0.5 which is the current version, Sierra is the only compatible Mac OS.
So unless you are prepared to upgrade to Sierra, Keynote 7 cant be installed.
If you have purchased versions of Keynote 6 or 7 previously, they can still be install ed from the Mac App Stor in the purchased tab section.
Keynote version 5.3 is available to purchase from Amazon as part of iWork 09, it runs perfectly well in all versions of Mac OS from Snow leopard to sierra, I have it running in El Capitan.