There is an issue if you have old files, I made a bug report already: https://github.com/letsencrypt/letsencrypt/issues/1888
Don’t use brew, use pypi aka pip install letsencrypt or you need to update your brew brew update
You could try sharing file via network, aka shared folder since samba should work fine. Or you could use vmware player and try it on it.
If you really wanted to pursue it, then boot from Linux Mint using rEFInd and do everything there, plug in usb drive and be done with it.
@peelman I also agree.