Try changing nameserver in the file /etc/resolv.conf to that of Google's 8.8.8.8:
sudo tee /etc/resolv.conf <<< $'nameserver 8.8.8.8'
And select repair when run below one:
pihole -r
If that doesnt help, run below one again:
sudo tee /etc/resolv.conf <<< $'nameserver 8.8.8.8'
And try upload a debug log with below:
pihole -d