You should consider to move to a current OS.
Debian 9 (Stretch) has reached EOL - it's Long Term Support ended on June 30, 2022.
Consequently, we have removed Debian 9 from the list of Pi-hole's supported OSs as early as July 7th.
We then adopted our build environments to reflect related changes and removed binary compatibility by July 31st, to be rolled out with the next Pi-hole release.
Prior to that release (i.e. the current September release), on August 29th, we released an advance announcement that we now -in addition to just notifying you that Debian 9 support has stopped- would ultimately drop support altogether: