What does this mean, Instance expected, got Vector3?

The others explained your question (by the way, Roblox discourages using the 2nd parameter to parent something, as it isn't as efficient as assigning the Position first and then parenting it); I'm going to explain every other problem you may experience after fixing it:

1) When you click the button, it won't spawn the part in the correct position. This is because Vector3s, strings, numbers, and other basic values "copy", whereas tables/instances do not. Illustration:

local a = 1
local b = a -- this only copies the value of "a" (currently 1) into "b"
b += 1 -- this only alters the value of "b"
print(a, b) -- 1, 2

Compared to:

local a = Instance.new("Part")
print(a.Name) -- Part
local b = a -- This copies a *reference* to the Part (you still only have one Part); now both variables refer to the same object
b.Name = "PartB" -- This changes the name of the object
print(a.Name, b.Name) -- PartB PartB

-- But note that if you assign to the entire variable "b", you aren't altering "a":

b = Instance.new("Seat")
print(a.Name, b.Name) -- PartB Seat

To fix this, access the .Position of the HumanoidRootPart in the MouseButton1Click handler, not before

2) Your second click won't spawn a part, even if it was 10 seconds after the first click. Think through your debounce implementation and what happens each step of the way after clicking on the button. Your script won't start waiting until you click for the 2nd time.

Improved script (I'm assuming it's in in a GUI that respawns when the player does):

local button = script.Parent
local root = game.Players.LocalPlayer.Character:WaitForChild("HumanoidRootPart")
local debounce = false

script.Parent.MouseButton1Click:Connect(function()
  if debounce then return end
  local part = Instance.new("Part")
  part.Position = root.Position
  part.Parent = workspace
  debounce = true
  wait(0.5)
  debounce = false
end)

if Position then Instance.new("Part").Position = Position end

for line 7. youre parenting the part to the position bruh

Try this:

local button = script.Parent
--[[ old 
local Position = game.Players.LocalPlayer.Character:WaitForChild("HumanoidRootPart").Position
-- new ]]
local HRP = game.Players.LocalPlayer.Character:WaitForChild("HumanoidRootPart")
debounce = false

script.Parent.MouseButton1Click:Connect(function()
    if debounce == false then

        local P = Instance.new("Part", workspace) -- should be put into the workspace and not the posiiton since this was causing the error
        P.Position = HRP.Position -- correct way to position a part (also changed the above Position variable to reference the `HumanoidRootPart` then only call the position when you click on this. This is done so it will update the position of the character. The old way would only update the position once the script has loaded and wont be on the character unless the character or player stays still)

        debounce = true
    else
        if debounce == true then
            wait(0.5)
            debounce = false
        end
    end
end)

Hope this helps!

Instead of this

Instance.new("Part", Position)

try this:

local Part = Instance.new("Part", game.Workspace)
Part.Position = Position