I created a tool, that has a handle inside of it followed by a Damage script so that if the tool touches a Humanoid it deals 25 damage, but how would I make the humanoid get knocked backward a few steps if the tool touched it?

Damage Script:

local player = game.Players.LocalPlayer
local handle = script.Parent.Handle
local debounce = false 

handle.Touched:Connect(function(hit)
	if not debounce and hit and hit.Parent.Humanoid then
		debounce = true
		
		hit.Parent.Humanoid:TakeDamage(25)
		wait(15)
		
		debounce = false
	end
end)

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If this is about causing knockback in a player, or an NPC you would clone from storage, something I like to do in this case is to have a bodyforce in the primarypart of the player or NPC before it gets put into the workspace. That way, you just access the NPC or player's bodyForce and apply a force to it whenever you want.

So it seems as if you are attacking NPCs, so to have a script place a bodyVelocity in an NPC the script would be like:

workspace.ChildAdded:Connect(function(child)
	if child.Name == "whatever the bodyforce is going to be in" then--this would be the model of the NPC
 		local bv = Instance.new("BodyVelocity")
		bv.Parent = child.PrimaryPart
	end
end)

Your script would look like:

local player = game.Players.LocalPlayer
local handle = script.Parent.Handle
local debounce = false 
local playerPos = player.PrimaryPart.CFrame

handle.Touched:Connect(function(hit)
    if not debounce and hit and hit.Parent.Humanoid then
        debounce = true
	local hitPos = hit.Parent.PrimaryPart.CFrame
        hit.Parent.Humanoid:TakeDamage(25)
	hit.Parent.PrimaryPart.bodyForce.Force = (playerPos-hitPos).unit*-3 --negative so it goes away from you
	wait(.5)
	hit.Parent.PrimaryPart.bodyForce.Force = Vector3.new(0,0,0)
        wait(14.5)--keeps the original 15 seconds, while allowing the force to stop after .5 seconds
	
        debounce = false
    end
end)

To explain the bodyForce line, it makes it so that Point A is you and Point B is the enemy, but it is multiplied by a negative number so that the enemy is knocked away from you while being in line with you.

To visualize this, you are the end of a stick, the enemy is the fulcrum (the point a lever rotates on) and if we multiplied the force by a positive number, the enemy would go towards you, if we multiply it by a negative number, the enemy goes to the opposite end of the stick.