You can do this by hooking into the edit-post.php
page. There are a variety of hooks you could use, for example, if you want to display a message below the edit title field, you could hook into edit_form_after_title
.
See the example code I have below with comments:
add_action(
'edit_form_after_title', // the hook
'wpse_228208' // the function
);
function wpse_228208() {
global $post;
// confirm if the post_type is 'post'
if ($post->post_type!== 'post')
return;
// you can also perform a condition for post_status, for example, if you don't want to display the message if post is already published:
if ($post->post_status!== 'publish')
return;
// here goes your message
echo '<div style="">Hello world!</div>';
}