Unable to download apps on iMac running macOS Catalina

Back up your data, check the application compatibility table, click here, and if your iMac is a late 2015 or newer model, then on the download link for a higher OS. Alternatively, check the developer's website for a version which will run on Catalina, or try downloading it to a more-recent Mac on the same Apple Account first.

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That's just what happens with old OS's, a way around is to already having purchased the app and hit download, it might ask you to download an older version of the app