@Noglutenoproblem It’s a mandatory 10 minute preview and normally the first lecture. You may want to add a short intro in the first section to take care of this requirement. The lecture will unlock so you can enable the preview.
You can make any lecture a "preview" lecture. I suggest you choose which lectures to include to make up the ten minutes you want to be available. But, there must be ten minutes of previews. If you don't choose them, Udemy will automatically choose the first lectures.