Why is this data store not working?

This script makes a GUI label visible if the player has 0 saved under CheckpointReached and sets it to 0 if the player doesn't have any data stored. This function is called when a button is pressed to view whether they had any data saved. However it doesn't work - there are no errors though. What has gone wrong?

function onClick()

--Find the Player
local player = script.Parent.Parent.Parent.Parent.Parent

--Find the DataStore value of CheckpointReached
local DataStore = game:GetService("DataStoreService"):GetDataStore("CheckpointReached")
local key = "user_" .. player.userId


-- If the value is nil set it to 0
if DataStore:GetAsync(key) == nil then
DataStore:SetAsync(key, 0)
end
--If the value is 0 then make SaveError visible
if DataStore:GetAsync(key) == 0 then
script.Parent.SaveError.Visible = true
end


end

script.Parent.MouseButton1Down:connect(onClick)