How can I make it so when a block is touched it changes a stat?

Player wasn't defined, try this instead:

local Brick = script.Parent
Brick.Touched:connect(function(hit)
	local Player = game.Players:GetPlayerFromCharacter(hit.Parent)
	if Player then 
		Player.leaderstats.Retrives.Value = Player.leaderstats.Retrives.Value + 1
	end
end)

and make sure you have something called "leaderstats" in your player, "Retrives" in leaderstats, and the script in your brick.