The reason you would get this error is most likely because of one of two reasons:
- The account you have on the site doesn’t have sufficient privileges. Your user role needs to be Author, Editor, or Administrator to be able to manage content in the app. You can find more info about this for self-hosted sites (WordPress.org) here and for WordPress.com here
- You recently changed the URL associated with your blog in the app, moved your website to a new host without logging out of the WordPress App, or other similar issues with connectivity directly to your site. In this case, try logging out of the WordPress App, and back in using new credentials.