My installs so far have been upgrade-installs from a previous v5. (I have never done a bare-metal install of V6 on the armel devices.
I know there is an env flag that you can use to suppress checking for OS... not sure how that works ...
OK, this thread gives some details but is rather old…
Maybe:
PIHOLE_SKIP_OS_CHECK=true pihole -up
IIRC, it may indeed install the web and core-scripts correctly and simply fail on the pihole-FTL binary.