If this is happening on a new page you are trying to create, check the page’s permalink slug (set to “plain” to troubleshoot). If you are already using the same slug name elsewhere, such as a post category or product category, and then assign the same name to a page, the permalink confusion can cause this error to occur. For example, if you have a category with a slug of “sports”, do not use “sports” as the slug for your page. Name your new page something else, and the problem will be resolved.