How do you make like a power up like walkspeed and max health when you touch a gui?

What incapaz said is correct. The gui usually loads before the player, so at the time the script ran, there was no LocalPlayer.Character

A hacky fix is to add wait()s to the beginning of your script, but that doesn't solve the problem. The proper way is to use some form of wait on the character. Here's one solution:

local db = true
local cooldown = script.Parent.Cooldown
local Kaioken = script.Parent
local player = game.Players.LocalPlayer
if not player.Character then
    player.CharacterAdded:wait()
end

local character = player.Character

if not character:FindFirstChild("Humanoid”) then
    character:WaitForChild(”Humanoid”)
end

local human = character.Humanoid

Remember that this is just one solution, and I'm sure there are better ways to do this. But that's for you to discover.

Even though you didn't ask, I'll fix the rest of your script, too.

Kaioken.MouseButton1Down:Connect(function()
    if db then
        db = false
        human.MaxHealth = human.MaxHealth +5
        human.Walkspeed = human.Walkspeed +5
        wait (5)
        human.MaxHealth = human.MaxHealth -5
        human.Walkspeed = human.Walkspeed -5

        for i = 5, 0, -1 do
            cooldown.Text = "Cooldown: "..i
            wait (1)
        end
        db = true
    end

end)