Hello and welcome to the forum 
Some Mac user found that connecting an external keyboard solve the issue. Apparently a mac keyboard is unable to change boot device in Zorin 16 before the installation. It will work after the installation.
to do a fresh install to Zorin 16, be sure to reboot to your usb drive by pressing and holding the ALT key on a non-Mac keyboard. It is working for me at least.
Actually we are just users like yourself.
None of us are related to Zorin group in any way.
As such, we are all volunteers here and NOT a hired customer service.