Help on how to use DataStores, retrieving and storing?

I'm trying to make a saving database of someone's department in a group, I have a GUI which successfully changes the Department depending on the entry, but then here's my hard part, how to save it to a DataStore and retrieve it afterwards.

My attempt (Store)

local DataStore = game:GetService("DataStoreService"):GetDataStore("Departments")
local key = "user_" .. script.Parent.Parent.userId

script.Parent.Parent:WaitForChild("leaderstats").Department.Changed:connect(function()
	DataStore:UpdateAsync(key,script.Parent.Parent:WaitForChild("leaderstats").Department.Value)
--This is where it stops working, "Unable to cast value to function"
end)

(Retrieve)

local group = 1234 --GroupID
local DataStore = game:GetService("DataStoreService"):GetDataStore("Departments")

game.Players.PlayerAdded:connect(function(player)
	local leadrsts = Instance.new("IntValue",player)
	leadrsts .Name = "leaderstats"
	local rank = Instance.new("StringValue",leadrsts )
	rank.Name = "Rank"
	local department = Instance.new("StringValue",leadrsts )
	department.Name = "Department"	
	local key = "user_" .. player.userId	
	department.Value = DataStore:GetAsync(key)
end)

Any help?

They're both server scripts