When you define a page as the main blog page, that page becomes the main blog index archive and will only show the latest posts, and will not display any content you have added to the page.
So there should not be an error message or any content other than just the standard latest posts.
In the page editor, you will still see the content you have added but it won’t be visible on the front-end.
Hope I have understood your question correctly? If not, please let me know the page URL and I’ll be able to take a look and advise further. Thanks.
