How to create event where when a player steps on a part, the part's transparency turns to 1?

There's a bit of a problem with your code, first off, you are running ChangeTransparency instead of connecting it, you are running the function and then connecting nothing (nil) to the event, another thing, you forgot to check if the touched object is a player (or if you want, a rig), to do this, you can check if a humanoid exists in the hit part parent, because the hit parameter only gives you a part, for example, if you touch a part with your left arm, hit will be your left arm and hit.Parent will be the character, if you want only player touching to make it transparent, you need to add another step, you can use GetPlayerFromCharacter to get the player or check if the touched character is a player, also you don't need a change transparency function lol

local players = game:GetService("Players") -- Useful for later
local part = workspace.ButtonTest -- the part that will change transparency, this is only for if you want more than one button, so you don't have to change the whole code to make another button
local transparencyToSetTo = 1 -- The transparency to set the part to on touch (optional)

part.Touched:Connect(function(hit)
	local humanoid = hit.Parent:FindFirstChild("Humanoid") -- We use FindFirstChild instead of a period (.) or a WaitForChild is because we don't know if hit.Parent has a humanoid, for example, if a part in the workspace touches the button, hit.Parent will be workspace and there will be no humanoid and the script will error and break
	if (humanoid) then
		-- if a character
		-- Just set the transparency here if you want, but if you don't want dummys or npcs to activate the button, put the next few lines
		-- part.Transparency = transparencyToSetTo
		local player = players:GetPlayerFrom(hit.Parent) -- this is to check if the character that touched the part has a player assigned to it
		if (not player) then
			-- If the character has no player, for example, npc or dummy
			return -- Don't let it go past this point, that's what the return is for
		end
		part.Transparency = transparencyToSetTo
	end
end)