Debian 9 is no longer supported. It is past EOL:
Debian 9 (Stretch): EOL 2020-07-06, EOL LTS 2022-06-30
You can skip the OS check with the following:
PIHOLE_SKIP_OS_CHECK=true sudo -E pihole -up
You may also consider updating your OS to something currently supported by Debian and Raspbian (Bullseye, for example).