If the file is meant to be a page, double check you have an `export default` with the React Component instead of an `export`. If you're already using `export default`, make sure the returned value is a valid React Component.
If you need to support a different file extension for a page component (such as `.mdx`) or would like to include non-page files in the `pages` directory, configure [Custom Page Extensions](/docs/pages/api-reference/next-config-js/pageExtensions) in the `next.config.js`.