i'm not anyone special here, but i think i can help.
This is not an adlist, it's a domain
You need to blacklist "adclick.g.doubleclick.net" as a domain, not an adlist.
An adlist is this -
https://raw.githubusercontent.com/StevenBlack/hosts/master/hosts
So you put this URL in "adlist" and pihole reads all of these domains.
You're telling pihole to read all the text at "adclick.g.doubleclick.net" when there isn't a list of domains at that URL
In Steven Black's list, "adclick.g.doubleclick.net" is already there, so adding it again will achieve nothing. It's already "activated" in pihole
ctrl + f in that github link and you'll see it's there. Or "Tools --> Search Adlist" in pihole GUI