A 404 error means that you are attempting to load a page that your Next.js app doesn’t have a route for. Next.js is failing to render this route. It could be that Next.js isn’t building for another reason and is unable to render any route. Review the server build logs in code mode to see if you are building successfully.
If your app is building successfully (I suspect it isn’t) then look at this log for which route is giving the 404 and ask Gemini to fix it.