Why does this coroutine fail to work correctly?

The hastag on line 5 is not necessary due to "cooldown" already being a number :)

wait()
function Cooldown(plr)
local stat = game.ServerStorage.PlayerStats2:FindFirstChild(plr.Name):WaitForChild("SummonCooldown")
local cooldown = stat
for i=1,cooldown.Value do
wait(1)
stat.Value = stat.Value - 1
end end

function game.ReplicatedStorage.CheckSummonCooldown.OnServerInvoke(plr,cooldown)
local stat = game.ServerStorage.PlayerStats2:FindFirstChild(plr.Name):WaitForChild("SummonCooldown")
if stat.Value == 0 then
stat.Value = cooldown
coro = coroutine.create(Cooldown)
coroutine.resume(coro,plr)
return 0
else
return stat.Value
end end

If you aren't using coroutine.yield in your code, you don't really have any business using the coroutine library.

Use spawn(f) if you want to start a new function "in the background". It plays more nicely with wait.

There's not a good reason for the for loop in the first place. Just wait(stat.Value) stat.Value = 0 accomplishes the same thing, but is much simpler.

In that case, you might as well just use a delay.

Instead of using :FindFirstChild(plr.Name) just use [plr.Name].

wait()

function game.ReplicatedStorage.CheckSummonCooldown.OnServerInvoke(plr,cooldown)
    local stat = game.ServerStorage.PlayerStats2[plr.Name]:WaitForChild("SummonCooldown")
    if stat.Value == 0 then
        stat.Value = cooldown
        delay(cooldown, function()
            stat.Value = 0
        end)
        return 0
    else
        return stat.Value
    end
end

and at that point SummonCooldown might as well be a BoolValue.