This post explains the cause of the problem: https://forum.audacityteam.org/t/ubuntu-mint-audacity-2-4-1-ppa-problem/57517/1
As described in that post, I would recommend installing the deb version from the main Ubuntu repository.
When you have done that, write back and say which version of Audacity you have, and which version of Ubuntu / Mint / other 'buntu based distro and we should be able to give you appropriate instructions for adding Nyquist plug-ins.