Are you sure you did the part " Make sure the ca-certificates-mono package is installed."?
A forum poster solved the warning on Linux by doing that. I don’t know macOS’ package tools.
EDIT 1:
Found another Linux user using this fix, and there might be more. macOS seems complicated.
2.0.3.7 on Mac: Missing certificates gets into it. I’m not sure why others aren’t seeing it though.
What does a mono --version say? Perhaps you could see if updating to current mono helps.
EDIT 2:
https://packages.ubuntu.com/eoan/mono-complete
shows the Ubuntu package names, where mono-complete has ca-certificates-mono and more.
I’m not sure what macOS package design looks like. There’s also a brew install mono plan.