How do you make the mouse lock on to a HumanoidRootPart?

First of all, Mouse.Hit.p is a Vector3 value, and not a CFrame value. Mouse.Hit is already a CFrame value, so if you're trying to set the Mouse's CFrame, you don't need the .p at the end.

Mouse.Hit = Mouse.Target.Parent:WaitForChild("HumanoidRootPart").CFrame

However, you cannot change the Mouse's Position, as they're a readonly property. You can change Mouse.Target.Parent:WaitForChild("HumanoidRootPart").CFrame to the Mouse's CFrame (Mouse.Hit) like this:

Mouse.Target.Parent:WaitForChild("HumanoidRootPart").CFrame = Mouse.Hit

but like I said, you cannot set the Mouse's position to another position via a script.

Also, it is more safe and effient to get your own Character from the Player (Player.Character) , rather than getting it from the Workspace (game.Workspace:WaitForChild(Player.Name)).

If you want to Lock On to a Character's HumanoidRootPart, what I would do is make my own HumanoidRootPart to look at the opponent's HumanoidRootPart. Like this:

local Player = game.Players.LocalPlayer
local Character = Player.Character
local HumanoidRootPart = Character:WaitForChild("HumanoidRootPart")

local Mouse = Player:GetMouse()

local UIS = game:GetService("UserInputService")
local LockedOn = false

UIS.InputBegan:Connect(function(Input, gameProcessedEvent)
    if gameProcessedEvent then return end
    if Input.KeyCode == Enum.KeyCode.E then
        if LockedOn == false then
            LockedOn = true
            if Mouse.Target.Parent:WaitForChild("Humanoid") then
        local opponentsRootPart = Mouse.Target.Parent:WaitForChild("HumanoidRootPart")
                while LockedOn == true do
                    HumanoidRootPart.CFrame = CFrame.new(HumanoidRootPart.CFrame.p, Vector3.new(opponentsRootPart.CFrame.x, HumanoidRootPart.CFrame.y, opponentsRootPart.CFrame.z)
                    wait(0.1)
                end
            end
        elseif LockedOn == true then
            LockedOn = false
        end     
    end
end)

One more thing, using a while loop multiple times is not ideal. In this case, you should use a repeat loop, as it only loops until a certain condition.

From this:

 while LockedOn == true do
    HumanoidRootPart.CFrame = CFrame.new(HumanoidRootPart.CFrame.p, Vector3.new(opponentsRootPart.CFrame.x, HumanoidRootPart.CFrame.y, opponentsRootPart.CFrame.z)
    wait(0.1)
end

to this:

repeat wait(0.1)
    HumanoidRootPart.CFrame = CFrame.new(HumanoidRootPart.CFrame.p, Vector3.new(opponentsRootPart.CFrame.x, HumanoidRootPart.CFrame.y, opponentsRootPart.CFrame.z)
until LockOn == false

This will repeat its loop every 0.1 second until the LockOn value is false.

I hope I helped in any way, and I hope you find your best solution.

You cannot control the position of the mouse directly.

That being said, a quick and dirty way of doing this would be to lock the player's camera to first person, and then point the camera at the part. You can then unlock the camera. This will be very jarring, but there's not much else that can be done.

i already have answered this question, but it's alright. u guys are the best for answering my question :D