Why doesn't my frame's visibility propertry set to true?

Hi! so the issue would be comming from line 5

local frame = game.StarterGui.ScreenGui.Frame

because that would not affect the GUI of your player, notice when you get in a game under Players you have PlayerGui so thats what you want to set the visibility

So firstly you have to get the Gui of the Player, not the Gui in StarterGui

local part = script.Parent
part.Touched:Connect(function(hit)
    local hum = hit.Parent:FindFirstChild("Humanoid")
    if hum then
		-- hit.Parent is the character so the line below gets player's folder from the character
		local p = game.Players.getPlayerFromCharacter(hit.Parent)
		
		-- so replace your frame variable with the player's ScreenGui
		local frame = p.ScreenGui.Frame

        if frame.Visible == false then
        frame.Visible = true
        end
    end
end)