SetCore ChatSystem issues?

Hi, I was attempting to make a join message using ChatMakeSystemMessage. I was having no issues testing it in studio, but when I started the server this error appeared.

SetCore: ChatMakeSystemMessage has not been registered by the CoreScripts

I've tried moving the script into various different places, and even changing it to a LocalScript, but no luck.

Is this error due to the fact that the player may load in before the CoreScripts do, or is it just something wrong with Roblox?

Edit: here is the code I'm using, it gets called on my a RemoteEvent which has no issues

function sendMessage(kill, dead)
	if dead then
		game:GetService("StarterGui"):SetCore("ChatMakeSystemMessage",{
		Text = "[Server]: "..dead.." was rekt by "..kill.."!"; -- Required. Has to be a string!
		Color = Color3.new(.8, .2, .2); -- Cyan: (0,1,1), defaults to white: Color3.new(1, 1, 243/255)
		Font = Enum.Font.SourceSansBold; -- Optional, defaults to Enum.Font.SourceSansBold
		FontSize = Enum.FontSize.Size18; -- Optional, defaults to Enum.FontSize.Size18
	})
	else
		game:GetService("StarterGui"):SetCore("ChatMakeSystemMessage",{
		Text = "[Server]: "..kill.." has joined the game!"; -- Required. Has to be a string!
		Color = Color3.new(1, 1, .68); -- Cyan: (0,1,1), defaults to white: Color3.new(1, 1, 243/255)
		Font = Enum.Font.SourceSans; -- Optional, defaults to Enum.Font.SourceSansBold
		FontSize = Enum.FontSize.Size18; -- Optional, defaults to Enum.FontSize.Size18
	})
	end
end