Sure! I will give it a shot!

Hi again, regarding this issue, I have realized something. Because we redirect to the certain URL res.redirect('/docs/${user}/${repo}/'); if the URL doesn't exist no matter what we have in my404.md, it doesn't shows. I have tried 2 different ways, one is use res.sendFile('index.html') when username or repo is invalid.

if(user && repo){
      res.redirect(`/docs/${user}/${repo}/`);
    }else{
      res.sendFile(path.resolve(__dirname, './docs/index.html'));
    }

In this case, we can show empty website with 404-page error message(I don't know where the error message came from) -cannot use notFoundPage: my404.md -. In this case, we need to make a proper 404 error page with html and send the static file when we come across the 404 error.

if(user && repo){
      res.redirect(`/docs/${user}/${repo}/`);
    }else{
      res.sendFile(path.resolve(__dirname, './docs/404page.html')); // like this
    }

Another is we can add notFoundPage: my404.md for every generateHtmlWithConfig() however in this case if the user or repo is invalid no data there. So we cannot make _doc directory properly. So it may require a lot changes on the program.
I suggest the first approach because it is simple and doesn't require a lot changes.
Please let me know if there is another way to fix it!!!!
I am not expert about docsify, so there should be better way I believe!!!