How do I clone the player's character and have it appear on my ViewportFrame?

Alright, so there's multiple problems with your script...

Your conditional statement (if-then) is a bit incorrect. I don't think a CharacterAdded event would be a good idea to use in the statement, and you're searching for the humanoid in the player itself. The humanoid is in the character, not the player.

Then an error occurs. On line 9, clone is a model (player characters are models), and it doesn't have a position property. Instead, you should use the :MoveTo() function. It's pretty much the position property in the form of a function for models.

After that, another error occurs at line 16, and it's similar to the last error. clone doesn't have a position property, but :MoveTo() doesn't let you read the position, only change it. There's several ways to get the position, but I'm just going to get the HumanoidRootPart's position in the character.

Your script should look like this:

--Local Script
local player = game.Players.LocalPlayer
local character = game.Players.LocalPlayer.Character
local viewportFrame = script.Parent

repeat wait(1) until character ~= nil -- Waits for character to load

if character:FindFirstChild("Humanoid") then
	character.Archivable = true
	local clone = player.Character:Clone()
	clone:MoveTo(Vector3.new(0, 0, 0))
	clone.Parent = viewportFrame

	local viewportCamera = Instance.new("Camera")
	viewportFrame.CurrentCamera = viewportCamera
	viewportCamera.Parent = viewportFrame
	viewportCamera.CFrame = CFrame.new(Vector3.new(0,2,12),clone.HumanoidRootPart.Position)
end

Hope this helped! If you have any questions, just comment!