Investigate the list of page(s) specified in the error message.
For each, you'll want to check if the file is meant to be a page.
If the file is not meant to be a page, and instead, is a shared component or file, move the file to a different folder like `components` or `lib`.
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 valid is a valid React Component.