Your Operating System is not supported.
The debug log is showing the issue:
*** [ DIAGNOSING ]: Operating system
[✓] Distro: Raspbian
[✗] Version: 9
(...)
[✗] Error: Raspbian is supported but version 9 is currently unsupported
If you are using Raspbian 9 you won't be able to install Pi-hole (this OS uses an older GLIBC and there is no way to update this library).
You need a supported OS.
Reference: