How can I use data stores correctly?

local DataStore = game:GetService("DataStoreService"):GetDataStore("mnaj22 level")


game.Players.PlayerAdded:connect(function(player)
	wait()
	
	Level = player.PlayerGui.ScreenGui.Value
	
	local key = "player-" ..player.userId
	
	local savedValues = DataStore:GetAsync(key)
	
	if savedValues then
		Level.value = savedValues(1)
	else
		local valuesToSave = (Level)
		DataStore:SetAsync(key, valuesToSave)
	end
end)

I'm not well-versed with data stores as yet, so I know I'm not the most qualified to answer this question -- but no one else is doing so. From looking at it (and checking the wiki a bit), it doesn't look as if your problem is in using the storage itself, but that you're setting your 'Level.value' to 'savedValues(1)' rather than just 'savedValues'. Not sure if I'm just misreading it.

local DataStore = game:GetService("DataStoreService"):GetDataStore("mnaj22 level")


game.Players.PlayerAdded:connect(function(player)
    wait()

    Level = player.PlayerGui.ScreenGui.Value

    local key = "player-" ..player.userId

    local savedValues = DataStore:GetAsync(key)

    if savedValues then
        Level.Value = savedValues[1]
    else
        local valuesToSave = Level
        DataStore:SetAsync(key, valuesToSave)
    end
end)

Few syntax mistakes. When you want to access a member of a table, use square brackets [] not parentheses ().