If you receive this error message, it’s because you’re running an older operating system on your Mac, most likely OS 10.10 or 10.9.
You need to have at least OS 10.11 to run Football Manager 2020.
It’s free and easy to upgrade, giving you other benefits when doing so.
Guidelines on how to do this can be found here - https://www.apple.com/uk/macos/how-to-upgrade/
Once updated you will now be able to run the game on Catalina, but only after making sure you're running a 64-bit version of Steam. More details about how to to this here -