[Help!] Button to exchange Money for Levels using [DataStore] .Any way to fix this script?

UpdateAsync does not allow you to use anything that yields within it's function, and that's why you get that error. That also includes other Datastore calls.

You need rearrange your end statements so they are one after the other and not one within the other.

local LevelStore = game:GetService("DataStoreService"):GetDataStore("LevelStorageNEW")
local MoneyStore = game:GetService("DataStoreService"):GetDataStore("MoneyStorageNEW")
local Player = game.Players.LocalPlayer

script.Parent.MouseButton1Click:Connect(function()
    local key = "user_" .. Player.userId
    local key2 = "user_" .. Player.userId
    MoneyStore:UpdateAsync(key, function(oldValue)
        local newValue = oldValue or 0
        newValue = newValue - 100
    end)
    LevelStore:UpdateAsync(key2, function(oldValue2)
        local newValue2 = oldValue2 or 0
        newValue2 = newValue2 + 1
        return newValue
    end)
end)

However, using UpdateAsync is pointless here as you are updating player data, something SetAsync will do just fine as there is no risk a different server will change it.