You strike me as a tech-capable sort - do you have any advanced setups on this Mac, like running a VPN? This article has a list of issues known to interfere with proper installation. Would you read through those to see if any of them apply to you?
Before you try a re-install, you may want to do a thorough deletion of all the Zwift-related files already on that disk. This other article will walk you through the steps.
Would you let us know if those things solve the installation issues for you?