Why can I not install Keynote on my MacbookPro?
As mentioned at the bottom of https://www.apple.com/keynote/features/

Pages for Mac, Numbers for Mac, and Keynote for Mac are available on the Mac App Store. macOS Catalina or later required

so if you haven't installed Keynote before you may not be able to download it from the App Store.
Alternativly you can use the web version of Keynote on iCloud or try an application like LibreOffice.