The issue is due to the fact Pure OS does’t not support i386 packages. Purism support also confirmed we should be using Buster sources instead of stretch.
If you are using Debian sources, you should comment out all others such as Purism and Ubuntu with a #. Keep only the primary buster sources lists. Then do an apt-get update & proceed.
You can comment everything out except for these sources with Buster instead of Stretch. (Not sure where you got those source URL’s and why they are “unsigned”)
https://wiki.debian.org/SourcesList#A.2Fetc.2Fapt.2Fsources.list
I’ve taken a stroll down the rabbit hole of enabling i386 on Pure OS. It’s going to take time and effort to get it working.