We’re seeing this too on my private server (hosthavoc). One of the players on my server logged into an official server and was able to do fatalities just fine. I’m thinking it’s a server configuration problem, but I have no way of identifying what that problem is. ServerSettings.ini has

EnableFatalities=True

so it’s not that. I ran a local online game and was able to do fatalities, but there’s nothing else in my local serversettings.ini that looks related (there are very few differences overall other than some pvp settings).