The script does not accept a different key why?

If i replace uid_ with anything else it doesnt work anymore

local DSS = game:GetService("DataStoreService")

local datastore = DSS:GetDataStore("GeneralSaveData", "Players")

function generateDataKey(player)
    local ret = ("uid_" .. player.userId)
    return ret
end

function generateDataTable(player)
    local dataTable = {
        Units = player.leaderstats.Units.Value,
        Level = player.leaderstats.Level.Value,
        Xp = player.leaderstats.Xp.Value
    }
    return dataTable
end

function saveDataForPlayer(player)
    local key = generateDataKey(player)
    local data = generateDataTable(player)
    datastore:SetAsync(key, data)
end

function inputDataToPlayer(player, data)
    player.leaderstats.Units.Value = data.Units
    player.leaderstats.Level.Value = data.Level
    player.leaderstats.Xp.Value = data.Xp
end

function loadDataForPlayer(player)
    local key = generateDataKey(player)
    local data = datastore:GetAsync(key)
    inputDataToPlayer(player, data)
end

game.Players.PlayerAdded:connect(function(player)
    player:WaitForChild("leaderstats")
    loadDataForPlayer(player) --Load first thing when they join
end)

game.Players.PlayerRemoving:connect(saveDataForPlayer) --Save data when the player leaves the game