thumnail image in single post showing problem.

I think I saw the checkbox after I select a thumnail image that I can choose whether it has appeared on top of the post.

but after I updated I don’t see the checkbox and I can’t choose disappeared thumbnail in every single new post.

previous posts are showing that I wanted and checked not to show the thumbnail image.

but when I create a new one, it shows itself which is annoying to me. please let me know how to change. Thanks.