When you encounter mac os could not be installed, then it means that your Mac has encountered an error which led to the failure of the operating system installation process. Areas which you should examine when encountering this problem include storage space, startup disk, installer medium, network connection, and whether or not your Mac supports the particular operating system version you are trying to install. Should you experience the problem again even after rebooting your Mac, you will be able to access various tools via macOS Recovery, including Disk Utility for testing your startup disk as well as for reinstalling macOS. You should avoid erasing your Mac right away, especially when important files are still present in your Mac.
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