Trying To create a leaderbord that says the top player in the game but it errors?

Hello,

I have went through your code and made some modifications! Please read the commented code for the rundown of what I've changed and why I've made these changes to your script.

If anything seems wrong, feel free to leave a comment, or change my code yourself!

-- use datastore
ds = game:GetService("DataStoreService"):GetGlobalDataStore()

a = true
-- notice replacing highest with highestPlayer
highestPlayer = nil

-- turn "b" and "c" into one dictionary table; dic[player] = # survived
dic = {}
-- decompose repeated / reusable code into functions
function updateHighest()
    -- theres no need for "d" anymore
    -- however, we don't know if the highest value is old
    -- need a value to keep track of current ingame highest
    local current_highest = -1
    local current_highestPlayer
    for player, survived in pairs(dic) do
		print(player, survived)
        -- also, no need to wait() here; small list of players to go through
        if survived > current_highest then
            current_highestPlayer = player
			current_highest = survived
        end
    end
    -- highest = highest of current in-game players
    highestPlayer = current_highestPlayer
end

function loadUser(player)
    local survived = ds:GetAsync(player.UserId .. "Survivals") or 0
	print(survived)
    
    dic[player] = survived
end

-- use an event that runs a function when something happens to load this users' survivals
-- i.e. game.Players.PlayerAdded => new user joins
game.Players.PlayerAdded:Connect(loadUser)

-- main runtime
while a == true do
	updateHighest()
	if highestPlayer then
	    -- b[d] is now highestPlayer; c[d] is now dic[highestPlayer]
	    script.Parent.Text = (highestPlayer.Name .. ": " .. dic[highestPlayer])
	else
		print("nobody is here!")
	end
    wait(1)
end

Please note I did not add a Saving function, because I did not notice one in the original script. I highly encourage you to try making a Saving function using DataStore.