I might be wrong here, but since Pix is part of XApps, it wouldn't be available in the first party repos of any fistrobither than the Mint spins.

Perhaps you could add an external PPA:

sudo add-apt-repository ppa:embrosyn/xapps
sudo apt update
sudo apt install pix

Looks like it has been replaced, the closest I can find on KDE's website is Photos: https://apps.kde.org/koko/

Did you install/enable/make default flathub? I found Pix here, or you could also follow ZeStigs direction to add repositories, very useful to know how. Hope this helps and try not to use snaps, hehe, flatpaks are more reliable

edit: wrong pix, gotta read more

first three results on google for "pix linux" look useful