DataSave, why is this save returning nil?

If you want to save multiple values to a single DataStore, you should save them as a table:

carts = {"Red", "White", "Blue"}
test = {1,2,3}

local DSService = game:GetService('DataStoreService')
local Save = DSService:GetDataStore('Test')

game.Players.PlayerAdded:connect(function(player)
    pcall(function()
        player:WaitForChild("PlayerGui")
        Save:SetAsync(player.userId,{carts,test})
        wait(3)
	local saveData = Save:GetAsync(player.UserId)
        if saveData then
            for i = 1,#saveData[1] do
                print(saveData[i])	--> Red, White, Blue
            end
	    for i = 1,#saveData[2] do
                print(saveData[i])	--> 1, 2, 3
            end
        end 
    end)
end)

I’ve also found that using pcall can sometimes not work when you try to print, so if it doesn’t work try removing that.