Hi, and welcome!
The key information is
You don't have permission to change the installation in any way,
specifically, the directory /usr/local/texlive/2019/tlpkg/ is not writable.
Please run this program as administrator
There multiple dancers in this ballet but the principal dancer is texlive. I'm not sure why pandoc needs to update texlive for the tlpkg package, but it can't because it's running under a user id that doesn't have administrative privileges.
The best way to fix this without losing sanity to through the TexLiveUtility app to install tlpkg, so that pandoc doesn't have to be involved.
Please see the FAQ: What's a reproducible example (`reprex`) and how do I create one? Using a reprex, complete with representative data will attract quicker and more answers.