More notes from the trenches.
I recently attempted to install Plone 5.1.6 using the Unified Installer with a Python 2.7 conda environment. I'm assuming you know how to install conda.
Here are the key steps:
Step 1 - Get Xcode 11
Xcode 12 did not work with Catalina for me. It takes some searching but this is the direct link to older versions of Xcode: https://developer.apple.com/download/more/?=xcode
Step 2 - Get Homebrew and install dependencies
note: I'm not 100% sure about this step, perhaps step 3 below is enough to resolve the issue.
You can get it from https://homebrew.sh
Once I got homebrew, the dependencies I installed were to support the installation of Pillow:
brew install jpeg zlib
Step 3 - set the CPATH properly
export CPATH=`xcrun --show-sdk-path`/usr/include
(you can add this to your .zshrc or .bashrc)
You can read more about the issue here Cannot find zlib headers on macOS 10.15 Catalina
Step 4 - Create your Python 2.7 conda env
conda create --name=py2 python=python2.7
Step 5 - Use virtualenv with conda
conda activate py2 conda install virtualenv virtualenv venv source venv/bin/activate
Step 6 - Unpack the Unified Installer and install
Unpack the Unified Installer and install, you should be able to follow the standard *nix instructions from here.