The tool.Activated event does not pass a player who activated the tool to your listener. Thus, player is nil.

If this is in a LocalScript

If this is in a local script, you can use Players.LocalPlayer

local client = game:GetService("Players").LocalPlayer;
local hitbox = Instance.new("Part"); --// second parent argument is deprecated and causes performance issues if misused, parent object always last
	
--// assign other props here
	
hitbox.CFrame = CFrame.new(client.Character.HumanoidRootPart.Position);
hitbox.Parent = game.Workspace;

Another issue you have is you are indexing the new function itself and not calling it. And you cannot construct a CFrame out of a CFrame. You can, out of a Vector3 and numbers however.

CFrame.new(client.Character.HumanoidRootPart.Position);

Also R15 does not have a Torso but all body types have a HumanoidRootPart. Use that for compatibility purposes.

If this is in a Script

If this is in a server script, you can use tool.Parent to get the character when the tool is equipped.

here is an example;

local tool = script.Parent;

tool.Activated:Connect(function()
	
	local character = tool.Parent; --// This tool is going to be activated when equipped so this will be the character.

	--// Create your part down here
end);