Your layout or page inside the app directory exports an invalid field. In these files, you're only allowed to export a default React component, or Segment Configuration Options for layout and pages, such as revalidate, generateStaticParams, etc.

Other custom exports are not allowed to catch misspelt configuration options and prevent conflicts with future options.

You can create a new file and co-locate it with the page or layout. In the new file, you can export any custom fields and import it from anywhere.