Why is getchildren index nil?

The reason why your script is erroring is that you're trying to set the parent of EVERYTHING in the workspace, including the player's models, and you can't parent those to a viewport frame unless you make a clone of them.

In order to fix this, you can also make you if statement checks if there is no Humanoid inside of v. If you also want to have the player inside the viewport frame you can make the player's model Archivable property set to true. Setting the Archivable property to true allows the model to be cloned thus allowing you to parent it to the Map.

local Map = script.Parent:WaitForChild("Map")

game:GetService("RunService").RenderStepped:Connect(function()
    for i, v in pairs(Map:GetChildren()) do
        v:Destroy()
    end

    for i, v in pairs(game.Workspace:GetChildren()) do
		print(v)
        if v.ClassName ~= "Camera" and v.ClassName ~= "Terrain" and not v:FindFirstChildOfClass("Humanoid") then
            v:Clone().Parent = Map
        end
    end
end)