There's another community post with 404 debugging tips that might be helpful. Please give these solutions a try and let us know how it goes.

https://community.vercel.com/t/debugging-404-errors/437

Vercel Community

Debugging 404 Errors

Sometimes things don’t go as expected when deploying a website. If you see a 404 on your site, that means the page or resource couldn’t be found. There are...

A human should be around soon to offer more advice. But you can also get helpful information quickly by asking v0.

It looks like your project has no framework preset right now, which means Vercel doesn't know to serve this as a Next.js app. This is usually set on initial project creation, so it can happen if you create your project before building out the Next.js app

You can set it either in the dashboard by going to Settings -> Build + Deployment, then selecting the framework preset, or by adding a vercel.json file with the framework set to nextjs

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{
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  "$schema": "https://openapi.vercel.sh/vercel.json",
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  "framework": "nextjs"
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}