Where to add debounce function?

Debounce doesn't have to be in a function, and I normally don't use a debounce function because I use different kinds of debounce all the time. For this, I would assume you only want the coin sound to play at most once every second.

local deb = false --Your debounce variable. As with any variable, the name doesn't really matter.

function onTouched(hit)

    if deb then return end --if deb is set to true, this will exit the function without running any code.
    deb = true --sets your debounce. The function is now "disabled"

    local H = hit.Parent:FindFirstChild("Humanoid")
    if H then --~= nil is never necessary, due to the concept of Truthyness
        script.Parent.Coin:play()

        wait(.5) --This wait goes inside your if check because if the thing that caused the Hit event doesn't cause the coin sound to play, why disable it from playing?

    end

    deb = false --"enables" the function. If the coin sound actually played, this will happen half a second after the sound played. If it didn't, this will happen instantly.

end
script.Parent.Touched:connect(onTouched)

Heres an example

function onTouched(hit)
deb = false
H = hit.Parent:FindFirstChild("Humanoid")
if H ~= nil then
if deb = false then
deb = true
script.Parent.Coin:play()
else
deb = false
script.Parent.Coin:stop()
end
end
end
script.Parent.Touched:connect(onTouched)