"NPC that follows player, script has a part where I can replace walk animation and has an attack system with animation I can replace within script, tell me where it goes"

First, insert an R15 Dummy NPC model from the toolbox in Roblox Studio. Then, follow these steps:

1. In the R15 Dummy NPC object, select Humanoid.
2. Right-click on the Humanoid and select "Insert Object".
3. Add a Script and paste the code below.

Replace the animation IDs in the `walkAnimationId` and `attackAnimationId` variables with the asset IDs of the animations you want to use.

```lua
local Players = game:GetService("Players")
local RunService = game:GetService("RunService")

local npc = script.Parent
local humanoid = npc.Humanoid
local root = npc.PrimaryPart

local walkAnimationId = "rbxassetid://YOUR_WALK_ANIMATION_ID"
local attackAnimationId = "rbxassetid://YOUR_ATTACK_ANIMATION_ID"

local walkAnimation = Instance.new("Animation")
walkAnimation.AnimationId = walkAnimationId
local attackAnimation = Instance.new("Animation")
attackAnimation.AnimationId = attackAnimationId

local function findNearestPlayer()
	local playerList = Players:GetPlayers()
	local nearestPlayer = nil
	local distance = nil
	local direction = nil 
	
	for _, player in pairs(playerList) do
		local character = player.Character
		if character then
			local distanceVector = character.PrimaryPart.Position - root.Position
			if not nearestPlayer then
				nearestPlayer = player
				distance = distanceVector.Magnitude
				direction = distanceVector.Unit
			elseif distanceVector.Magnitude < distance then
				nearestPlayer = player
				distance = distanceVector.Magnitude
				direction = distanceVector.Unit
			end
		end
	end
	
	return nearestPlayer, distance, direction
end

local function attackPlayer(player)
	local character = player.Character
	if character then
		local humanoid = character:FindFirstChild("Humanoid")
		if humanoid then
			humanoid.Health = humanoid.Health - 10
		end
	end
end

RunService.Heartbeat:Connect(function()
	local nearestPlayer, distance, direction = findNearestPlayer()
	if nearestPlayer then
		if distance < 5 then
			humanoid:LoadAnimation(attackAnimation):Play()
			attackPlayer(nearestPlayer)
		else
			humanoid:LoadAnimation(walkAnimation):Play()
			humanoid:Move(direction)
		end
	end
end)
```

This script will make the NPC follow the nearest player and attack them when they are within a certain distance. The walk and attack animations can be replaced by changing the asset IDs in the `walkAnimationId` and `attackAnimationId` variables.