currency DataStore wont save :( please help?

The way you save is wrong. It's gonna overwhelm the DatastoreService at times, and it's also gonna OVERWRITE everything that was set as the new one. (Like what you did, you saved Coins, but in the next line, you overwrite Coins with Rebirths.) It should be a table if you have multiple values.

And also the way that you load it is also wrong, like I said, using tables is best if you have multiple values to load.

Also, you should ALWAYS use pcall() if you're operating with Datastores. Only with the Async, though.

Here is my fix: (Untested)

local DataStoreService = game:GetService("DataStoreService")

local playerData = DataStoreService:GetDataStore("PlayerData")

local function onPlayerJoin(Player)
    local leaderstats = Instance.new("Folder", Player)
    leaderstats.Name = "leaderstats"

    local val1 = Instance.new("StringValue",Player)
    val1.Name = 'GotPet'
    val1.Value = ''

    local val2 = Instance.new("StringValue",Player)
    val2.Name = 'OpenValue'
    val2.Value = ''

    local Gems = Instance.new("IntValue", leaderstats)
    Gems.Name = "Gems"
    Gems.Value = 0

    local Rebirths = Instance.new("IntValue", leaderstats)
    Rebirths.Name = "Rebirths"
    Rebirths.Value = 0

    local Coins = Instance.new("IntValue", leaderstats)
    Coins.Name = "Coins"
    Coins.Value = 0

    local playerUserId = "Player_" .. Player.UserId  --Gets player ID

    local Success, Result = pcall(function()
        return playerData:GetAsync(playerUserId)
    end)

    if Result then
        print("Data found!")

        Coins.Value = Result[1] -- // Since in the saving part, Coins are the one that is set as the first value of the table, we set Coins as [1]
        Rebirths.Value = Result[2]
        Gems.Value = Result[3]
    else
        print("DataStore working but No data found")
        warn(Result)
    end

end

local function onPlayerExit(player)  --Runs when players exit



    local success, Err = pcall(function()
        print("Data Saved successfully")

        local playerUserId = "Player_" .. player.UserId

        playerData:SetAsync(playerUserId, {player.leaderstats.Coins.Value, player.leaderstats.Rebirths.Value, player.leaderstats.Gems.Value}) -- Now set as a table instead of individually

    end)



    if not success then
        warn(Err)
        warn('Could not save data!')
    end
end

game.Players.PlayerAdded:Connect(onPlayerJoin)

game.Players.PlayerRemoving:Connect(onPlayerExit)