Code:
local player = game.Players.LocalPlayer
local camera = workspace.CurrentCamera
player.CharacterAdded:Wait()
player.Character.WaitForChild("Head")
camera.CameraSubject = player.Character.HumanoidRootPart
camera.CameraType = Enum.CameraType.Attach
camera.FieldOfView = 70
game:GetService(("RunService")).Stepped:Connect(function()
camera.CFrame = CFrame.new(player.Character.HumanoidRootPart.Position) * CFrame.new(0, 0, 10)
end)

4 comments
do you get any errors in the developer console while play testing? (press F9 to see the console)
the errors might sometimes explain why something doesn't work.
the error is: Expected ':' not '.' calling member function WaitForChild
that means it should be player.Character:WaitForChild("Head"), the script wanted a colon ":" and not a dot "."