DataStoreService - Data not saving on leave?

local function SaveGameData(plr, dataToSet)
	local data
	if dataToSet then
		data = dataToSet
	elseif dataToSet == nil then
		data = Server[plr]
	end
	print(data, Server[plr], dataToSet)
	local success, err = pcall(function()
		return LocalData:SetAsync('UserID_' .. tostring(plr.UserId), data)
	end)
	if success then
		print(plr.Name .. ': Saved')
		Server[plr] = nil
	else
		error(err)
	end
end

When the player joins, everything works fine but on leave (data, Server[plr], dataToSet) are all printed nil.

Not sure how to fix this exactly.

That's because you haven't used the function PlayerRemoving This event is used for when the players left the game And make sure you use EnableStudio Api

local DATA_STORE_NAME = "Config"
local DATA_STORE_SCOPE = "MyGameName"
local KEY_NEW_FEATURE = "NewFeatureEnabled"

local DataStoreService = game:GetService("DataStoreService")

local dsConfig = DataStoreService:GetDataStore(DATA_STORE_NAME, DATA_STORE_SCOPE)

local function checkFeature()
    local isFeatureEnabled
    local success, err = pcall(function()
        isFeatureEnabled = dsConfig:GetAsync(KEY_NEW_FEATURE)
    end)
    if success then
        print(KEY_NEW_FEATURE .. ": " .. tostring(isFeatureEnabled))
        if isFeatureEnabled == true then
            print("Feature is enabled!")
        elseif isFeatureEnabled == false then
            print("Feature is disabled!")
        elseif isFeatureEnabled == nil then
            print("Feature is not set!")
        else
            -- Some other value was found in this key
        end
    else
        print("Failed to load feature! Error: " .. tostring(err))
    end
    return success
end

local function onFeatureChanged(isFeatureEnabled)
    print("Feature toggled: " .. tostring(isFeatureEnabled))
end

-- Listen for changes
dsConfig:OnUpdate(KEY_NEW_FEATURE, onFeatureChanged)

local function setFeatureEnabled(isFeatureEnabled)
    local success, err = pcall(function()
        dsConfig:SetAsync(KEY_NEW_FEATURE, isFeatureEnabled)
    end) 
    return success
end

setFeatureEnabled(true)
checkFeature()