In your 404.js page you added getInitialProps or getServerProps which is not allowed as this prevents the page from being static and 404.js is meant to provide more flexibility for a static 404 page. Having a static 404 page is the most ideal as this page can be served very often and if not static puts extra strain on your server and more invocations for serverless functions which increase the cost of hosting your site unnecessarily.
Remove getInitialProps from 404.js and make sure no HOC's used in 404.js attach getInitialProps.
If you want to fetch data for your 404.js page move it to a client side fetch inside of componentDidMount or useEffect(() => {}, []).