Your client does not have permission to get URL / from this server. That’s all we know.
I get this error message...even if I disable pihole for a few min. I have selected cloudflare and pihole also does my DHCP. The problem exists from all computers, users within the network....
Not a Pi-hole issue: The web server of the domain you are trying to access is telling you that you lack the permission to do so.
Note that this also means that name resolution was successful, or your client wouldn't have been able to receive that HTTP response status from the web server in the first place.
You'll need to fill out the template that is provided for us to be able to help you further.
A debug token is necessary, and screenshots of the query log showing the domains and what the response was would be helpful. Or the IP returned for those domains.