Follow the instructions in the regular script. Most of your logic in the module script was fine, you just had some small issues which I fixed.

Also, instead of SetAsync() consider using UpdateAsync() to avoid dataloss.

Message me on discord if you need anything else as I added you.

Regular Script:

module = require(game.ReplicatedStorage.ModuleScript)

game.Players.PlayerAdded:Connect(function(player)
    --gets data from datastore / creates new data entry in DS depending on if the player has played before
    -- and adds player to sessionData table onenter
    module.setupPlayerData(player)

    --Whenever you want to add data do the following: 
    module.ChangeStat(player, "Cash", 10)

    --prints the player's data. you might want a function called getData(player) to make this easier to read and so you can have sessionData as a private variable
    print(module.sessionData["Player_"..tostring(player.UserId)].Cash)

    --Whenever you want to update the leaderstat, do: 
    --(localtion of the leaderstat value).Value = module.sessionData["Player_"..tostring(player.UserId)].Cash

    -- ^^ this is why getData() would be useful so we don't have a messy line of code like this
end)

--TODO: call module.saveOnExit(player) on PlayerRemoving event

--this goes at the end of the script since it's an infinite loop
module.autoSave()

Module Script

local module = {}
local DataStoreService = game:GetService("DataStoreService")
local playerData = DataStoreService:GetDataStore("PlayerData")
local AUTOSAVE_INTERVAL = 120

--empty table to add player data to 
module.sessionData = {}

--function for changing a specific stat
function module.ChangeStat(player, statName, value)
    local key = "Player_"..player.UserId
    module.sessionData[key][statName] = value
end

--save the player data
local function savePlayerData(key)
    if module.sessionData[key] then
        playerData:SetAsync(key, module.sessionData[key])
    end
end

--set up player data
function module.setupPlayerData(player)
    local key = "Player_"..player.UserId
    local dataFromDS = playerData:GetAsync(key)

    if dataFromDS then
        module.sessionData[key] = dataFromDS
        print("loaded data from datastore")
    else
        module.sessionData[key] = {Cash=0, Subscribers=0}
        savePlayerData(key)
        print("created new data for player")
    end
end

--saves the player data on exit
function module.saveOnExit(player)
    local key = "Player_"..player.UserId
    savePlayerData(key)
end

function module.autoSave()
    print("started autosave function...")
    while wait(AUTOSAVE_INTERVAL) do
        print("autosaving...")
        for index, playerData in pairs(module.sessionData) do
            savePlayerData(index)
        end
    end
end

return module