Originally Posted by steadfasterX
mixing pip and the package manager files will make things worse.
installing multiple variants will make things worse, too.
so REMOVE all those mixed things you installed first.
Then read the OP again as it states CLEARY for *buntu, Debian (so Mint as well):
sudo pip3 install zstandard
i get this in Ubuntu Requirement already satisfied: zstandard in /usr/local/lib/python3.7/dist-packages (0.12.0)
Requirement already satisfied: cffi>=1.11 in /usr/local/lib/python3.7/dist-packages (from zstandard) (1.12.3)
Requirement already satisfied: pycparser in /usr/local/lib/python3.7/dist-packages (from cffi>=1.11->zstandard) (2.19)
but salt is still giving fatal error about zstandard