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That's a command for Terminal. Do a search for Terminal.app and run it. Then type: export SYSTEMVERSIONCOMPAT=1 and press return. That's telling your system that whenever an application asks it for the value of "SYSTEMVERSIONCOMPAT", respond with the number 1. This worked for me as well. Thanks to @billxiong! EDIT: I should note that I first had to install the app from the dmg I downloaded from the Parallels website. Once I opened the DMG and was presented with the Instal.app on screen, I ran the following in the Terminal: open -a '/Volumes/Parallels Desktop 15/Install.app' This successfully ran the installer. Now that Parallels is in my Applications folder, I can now run it by typing: open -a 'Parallels Desktop'