I was having the same issues with dual booting on an Intel Mac, and when I reached out for help, Zorin said, "Unfortunately due to the inclusion of Apple's T2 security chip in your model of Mac, it won't be possible to install any operating system other than macOS onto the computer's internal SSD.
However, you should still be able to install Zorin OS to an external hard drive and boot from it to use Zorin OS on your Mac.
In order to install Zorin OS to an external hard drive from your Mac, please follow these steps:
- Open the macOS Startup Security Utility by following the steps on this page: About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support
- Set the "Secure Boot" option to "No Security" and the "External Boot" option to "Allow booting from external media".
- Reboot the computer with the Zorin OS Installation USB drive plugged in
- Select the "Try or Install Zorin OS" option from the boot menu.
- When you start the installer, please make sure that the external hard drive you wish to install Zorin OS on is plugged in. Follow the on-screen instructions.
- When you reach the partitioning phase of the installer, please select the "Something else" option to open the manual partitioner and select to install Zorin OS to the external hard drive. Before using the "Something else" option at the partitioning phase of the installer, we would recommend you to look at this video which gives a comprehensive guide on how to use this option: How to use Partiton Editor in Ubuntu Installer - YouTube"
I found this handy list and confirmed that my Mac is indeed on it. Mac models with the Apple T2 Security Chip - Apple Support
Apparently you can force it, but this tutorial and the follow-up made me decide to just switch to a Linux laptop instead. HOW TO: Dual Boot macOS and Linux + Using Linux on a Mac. A Year Later...