Question multiple pages in jupyter notebook

Hi, everyone, I have an issue with my code. I cannot fix it. I use jupyter notebook to design a mutiple page in dash. I follow the steps of the dash website. I created app.ipynb, pages folder, and three pg.ipynb files. But this web-based app only shows the “404-Page not found” and “Python Multipage App with Dash”. I don’t know what the mistake is in my code. Please help me fix my problem. Thank you!