How do I make a loop out of this? [closed]

Your problem is that you're not reactivating your bool switch - or officially known as debounce.

This will cause your script to never pass the condition on line 4.

So to fix? Reactivate your debounce.

local Bool = false

script.Parent.ClickDetector.MouseClick:connect(function()
    if (not Bool)then --You checked if it's false
        Bool = true --Then set it to true, so the event won't re-iterate.
        local grass = game.Lighting.Grass:Clone()
        grass.Parent = workspace
        script.Parent.BrickColor = BrickColor.new("Really black")
        wait(1)
        script.Parent.BrickColor = BrickColor.new("Dark green")
        Bool = false --Reacitvate it!
    end
end)

More info on debounces here.

You have Bool acting as a sort of debounce for this function.

You require that Bool is false (ie not Bool is true) in order for your script to run -- but once you've made on blade, you set Bool to true (and nothing sets it to false later).

I'm guessing what you meant to do was to disable making new ones while it's changing from black to green -- and then enable it again.

enabled = true -- Use a better name than "Bool"

script.Parent.ClickDetector.MouseClick:connect(function()
    if enabled then
        local grass = game.Lighting.Grass:Clone()
        grass.Parent = game.Workspace
        script.Parent.BrickColor = BrickColor.new("Really black")
        enabled = false
        wait(1)
        script.Parent.BrickColor = BrickColor.new("Dark green")
        enabled = true
    end
end)

If you just want to know if there's already a part named "Grass" in the workspace, there's no need for a debounce at all:

script.Parent.ClickDetector.MouseClick:connect(function()
    if not workspace:FindFirstChild("Grass") then
        local grass = game.Lighting.Grass:Clone()
        grass.Parent = game.Workspace
        script.Parent.BrickColor = BrickColor.new("Really black")
        wait(1)
        script.Parent.BrickColor = BrickColor.new("Dark green")
    end
end)