Custom AMP Editor lets you control the output of the content on AMP. It helps you to customize a page/post according to your requirement to display on the AMP version.
If you want to use Custom AMP Editor, then first tick on ” Use this Content as AMP Content “. If you Don’t tick that, Custom AMP Editor will not work on that page/post.
You can find this option below your post editor or page editor. See this image: