Between lines 4 and 5 (or in line 19 before you call chooseClass(v)) you need to check the local player's value (probably in player.leaderstats .Rank.Value) for the proper value.

You will probably want to set up a table to keep track of the level requirements, like this:

levelRequirements = {1, 2, 4, 10, 20, 100} --level required for class1, class2, etc

Between lines 4 and 5 put this:

    local classNum = tonumber(class.Name:sub(6))
    if player.leaderstats.Rank.Value < levelRequirements[classNum] then
        --do something to show the player that they don't have the level for this class
        return
    end

(Don't forget to change player.leaderstats.Rank.Value to reflect whatever you've got setup in your game)

It's because you're making it wait 600000 SECONDS?! (or almost 7 days).. In other words you pause the script (thread, more precisely) for that much time.

Anyway, why do you need wait 7 days?!

If you really have a reason to, here are a bunch of ways to fix it

1. Use a separate thread using coroutines

-- Examples
function chooseClass(class)
    coroutine.resume(coroutine.create(function() -- THIS
        -- ............
    end))
end

function chooseClass(class)
    coroutine.wrap(function() -- THIS
        -- ............
    end)()
end

2. Use a separate thread using the spawn function

-- Examples
function chooseClass(class)
    spawn(function() -- THIS
        -- ............
    end)
end

-- ... OR
for i, v in pairs(script.Parent.Main:GetChildren()) do
    v.MouseButton1Up:connect(function ()
        spawn(function() -- THIS
            chooseClass(v)
        end)
    end)
end

3. Use the delay function

Instead of..

script.Parent.Main.Visible = true
wait(600000)

Do..

delay(600000, function()
    script.Parent.Main.Visible = true
end)