I need to create a flyer for an online store, which I want to put in each box with an order, but I cann't find the mockup of flyer. Maybe you have something that helped me?

You can find all the necessary information for this on the Internet. I know that there are many detailed video tutorials on how to use different image editors. You can also use ready-made materials, for example, here https://www.ls.graphics/free-mockups/flyer-mockup are free mockups of flyers, I think that you can use these mockups to create your flyer or you can use these mockups as a template with which you can create a flyer with your sizes. It seems to me that this is very convenient for a beginner in this, besides you will possibly improve your skills.