Script Not giving money to the player every 15 seconds?

Hi, I have noticed a few mistakes in your code

1. You have put a while loop, and then put a while loop after it. The first while loop would run infinitely. As a result, the other while loop would not be executed. To fix this, you can add a coroutine. Coroutines are used to make 2 or more things to execute at the same time. In your case, coroutines are relevant. We can simply wrap each of your loops with coroutine.wrap(). In the end, we simply call the two coroutines.

1. How you use Instance.new may be the cause of problems with performance. I have given an example that creates a part.

local part = Instance.new("Part")  
part.Name = "New Part" 
part.Parent = workspace

As you have noticed in that small piece of code, you use the Instance.new class, then, change the property of that new class, and then, it changes the parent. This decreases latency in your game.

As for 3, there were a few minor mistakes/tweaks that I will not include here. You might notice these changes.

Code:

local DataStore = game:GetService("DataStoreService"):GetDataStore("Stats")
local SaveTime = 15 -- Saves every (SaveTime) seconds (I recommend not going under 60 seconds...)

game.Players.PlayerAdded:Connect(function(plr)
    local PlayerKey = "user_".. plr.userId
    local stats = Instance.new("IntValue")
    stats.Name = "leaderstats"
    stats.Parent = plr
    print(stats.Name)
    if DataStore:GetAsync(PlayerKey) then
        print("Loading " .. PlayerKey)
        local Data = DataStore:GetAsync(PlayerKey)
        local RDCash = Instance.new("IntValue")
        RDCash.Name = "RDCash"
        RDCash.Value = Data.RDCash
    RDCash.Parent = stats
    else -- Setups new data -- I don't know why you put "else do" instead of just putting "else"
        local NewStats = {RDCash = 0}
        local RDCash = Instance.new("IntValue")
        RDCash.Name = "EXP"
        RDCash.Value = 250
    RDCash.Parent = stats
        DataStore:SetAsync(PlayerKey,NewStats)
    end
    local saveData = coroutine.wrap(function()
        while true do --Save loop
            wait(SaveTime)
            local PlayerKey = "user_".. plr.UserId
            local leaderstats = plr.leaderstats
            local Data = {}
            Data.RDCash = leaderstats.RDCash.Value
            DataStore:SetAsync(PlayerKey,Data)
            print("Saved Data")
        end
    end)
    local addCash = coroutine.wrap(function()
         while true do
            plr.leaderstats.RDCash.Value = plr.leaderstats.RDCash.Value + 15
            print("Done!")
            wait(3)
          end
     end)
   saveData()
   addCash()
end)