Coin value display text not working, what's the problem?

Pretty sure you just made small errors. Try this script. If it doesn't work, don't thumbs down. Tell me the error and see if I can fix it.

Note: Make sure to use a localscript for this. "game.Players.LocalPlayer" is only accessible through one.

while wait(10) do --loops this code every 10 seconds
    local player = game.Players.LocalPlayer --use this instead of what you had before, if the hierarchy gets messed up it will break .
    local coins = player.StarterGear:WaitForChild("Coins") --waits for "Coins", located in the player's startergear.
    script.Parent.Text = coins.Value .. "MC" --displays text on the gui.
end

1. Don't set properties as variables, but the objects:

a = script.Parent.Parent.Parent.Parent.Parent.Parent.StarterPack.Coins
script.Parent.Text = a.Value.."MC"

2. Use changed event and not a while true loop to update the value, it will eat less resources:

a = script.Parent.Parent.Parent.Parent.Parent.Parent.StarterPack.Coins
a.Changed:connect(function()
    script.Parent.Text = a.Value.."MC"
end)

3. Using local on variables eat less resources:

local a = script.Parent.Parent.Parent.Parent.Parent.Parent.StarterPack.Coins
a.Changed:connect(function()
    script.Parent.Text = a.Value.."MC"
end)

4. Why starterpack? On ServerScriptService, insert a script, which contains the code:

game.Players.PlayerAdded:connect(function(player)
     local val = Instance.new("IntValue")
     val.Name = "Coins"
     val.Parent = player
     val.Value = 500000
    player.PlayerGui.ScreenGui.fefe.fefhtuy.ggrgd.af.Text = val.Value.."MC" -- change the hierarchy by yourself
    val.Changed:connect(function()
        player.PlayerGui.ScreenGui.fefe.fefhtuy.ggrgd.af.Text = val.Value.."MC" -- change the hierarchy by yourself
    end)
 end)