Connect iPad as a second screen to MacBook

I can't connect my iPad as a second screen to my MacBook. I went threw all the steps in this link: https://support.apple.com/en-is/HT204095. In the last steps it says that the iPad should appear under the USB Device Tree, but mine doesn't. If it does not appear it says to contact apple support, so is there something you can do to help me with this?