I don't know about building Armory on a R-Pi. I've always cross compiled it for Raspbian. You can lookup R-Pi cross compiling quite easily. Be warned, it's painfully annoying to get it to work. I may just buy a Pi4 and build straight on that.
This OS is still on beta stage, and I suppose it also uses the latest python3 (not sure if it's also on QT5 or still QT4). It's therefore very likely the old stable branch of Armory couldn't run on it, and I should try compiling it from the development tree.
dev branch just went through the py3/qt5 migration, it's still quite unstable at this time. It doesn't build without manual intervention atm, you'd have to wait a little. My current near term project is to create and sign a mainnet tx with it.