How do i show my screen GUI message?

You wouldn't use StarterGui for this, StarterGui is just the starter gui. When a player joins, the StarterGui is cloned into their PlayerGui. Also with this script you should make sure to check if what touched the part is a player.

script.Parent.Touched:connect(function(part) --'part' is what touched script.Parent
    if part.Parent:FindFirstChild("Humanoid") then --this checks if it is a character that touched it by checking for a "Humanoid" which is in all characters
        local player = game.Players:FindFirstChild(part.Parent.Name) --this would get the player using part.Parent.Name which would be the character's name
        if player then --this just checks if the player exists, because if your game has any npcs the character would exist but no player with its name
            player.PlayerGui.ScreenGui.Frame.purchaseText.TextTransparency = 0 --this would change "ScreenGui.Frame.purchaseText.TextTransparency" in the player to 0
        end
    end
end)