FTL fails to download

One addition, if the pihole command is not installed yet "pihole: command not found", try do the same before running the installer.
And add the -x argument to the install command for verbose output:

sudo tee /etc/resolv.conf <<< $'nameserver 8.8.8.8'

curl -sSL https://install.pi-hole.net | bash -x

If this fails, could you post output for the install command (might want to redact some of the output) ?