Where Can I download an older version of the KeyNote for MacOS?
Keynote can’t be installed on “Macintosh HD” because macOS version 10.15 or later is required.
MacBook Air 13″, macOS 10.13
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If you have downloaded Keynote in the past, go to the App Store located in the Applications folder, log in (Store > Sign In) and click on the Purchased tab in the tool bar.
If you have never installed Keynote from the App Store, you will not be able to download Keynote.
Alternatively, if your Mac is compatible with Mac OS Catalina (Mac OS 10.15), you could install Catalina. If that is of interest, post back as there are a number of issues you will need to consider.