✓ Accepted answer
Many errors there. I'll go through them.
1: 'Game' should be lowercase
2: 'leaderstats' isn't a child of Players, use game.Players.LocalPlayer:FindFirstChild('leaderstats') to access your own leaderstats
3: this should be a localscript, in StarterGui.. then access the part with workspace.Part
4: Use a Changed event to change the part's transparency, rather than a loop. Much more efficient.
wait(1)
local leader = game.Players.LocalPlayer:FindFirstChild('leaderstats')
local part = workspace.Part
if leader.Quest.Value == 0 then part.Transparency = 0 else part.Transparency = 1 end
leader.Quest.Changed:connect(function(change)
if change == 0 then
part.Transparency = 0
else
part.Transparency = 1
end
end)