Why when I click it on low health I don't heal?

If you take a look at the documentation for the MouseClick event you can see that it is documented as.

RBXScriptSignal MouseClick (
   Player playerWhoClicked
)

This means that the event will return the player that clicked it (not a body part like the Touched event). We can use that to change your script around a little bit.

-- ... the rest of your script
function ClickToHeal(player)
    local character = player.Character
    if character then
        local Humanoid = character:FindFirstChild("Humanoid")
        local currentHealth = Humanoid.Health
        local newHealth = currentHealth + HealAmmount
     end
end

HealButton.ClickDetector.MouseClick:connect(ClickToHeal)

Now we get the player that clicked, check that they have a character (sometimes they don't), and then we change the humanoid around (I don't think then we have to check for it).