Pi-hole is probably incompatible with Debian Stretch.
The support was dropped more than one year ago: Upcoming Pi-hole changes – Pi-hole and Bye Bye Debian Stretch :: Looking for testers
Also, the there are 5 and a half years between v3.2.1 (Dec 22, 2017) and the current v5.17.1 version (May 30, 2023).