As we continue to upgrade Rocket League with new technologies, it is no longer viable for us to maintain support for the macOS and Linux (SteamOS) platforms. As a result, the final patch for the macOS and Linux versions of the game will be in March. This update will disable online functionality (such as in-game purchases) for players on macOS and Linux, but offline features including Local Matches, and splitscreen play will still be accessible.
Os Not Supported Epic Games
If you purchased Rocket League for Mac or Linux on Steam, the game will still work with full functionality when installed and played on a computer running Windows 7 or newer.
Well to me it sounds like GTA V does not support Mac OS. But in any case for support for this you should contact Epic Games From what I can tell this is the supported OS for GTA V from the epic launcher. To get a copy of macOS Mojave to install on an unsupported Mac, you have to be in the Apple Beta program but more importantly than that, you have to download it on a supported Mac.
What Does Os Unsupported Mean On Epic Games
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Additional information can be found in the support article here.