The debug log shows:
[✗] Error: Raspbian is supported but version 9 is currently unsupported (https://docs.pi-hole.net/main/prerequisites/)
The problem is "Stretch" (Debian 9 / Raspbian 9) is old and doesn't have all required libraries to execute the current version of pihole-FTL.
Also on the debug log:
pihole-FTL[474]: /usr/bin/pihole-FTL: /lib/arm-linux-gnueabihf/libc.so.6: version `GLIBC_2.28' not found (required by /usr/bin/pihole-FTL)
This was announced in 2022-08-29 here:
And here: Upcoming Pi-hole changes