Can't install keynote

When I'm trying to install keynote from the App Store, the following message will appear: 'We couldn't complete your purchase' 'Keynote cannot be installed on 'Untitled' because macOS version 10.14 or higher is required.'

Currently I'm using a macbook air (early 2014) running Mac OS High Sierra 10.13.6. and I can't update it to 10.14, because I can't find the download in the App Store, only macOS Catalina. Does somebody now how to fix this problem?