Eye-catch image is not displayed

I'm creating a site from 1 with WordPress.
I am trying to set the eye catch image on a fixed page, but there is no display.
When I did a search, it appears when I check the credentials in the display option, but I can't find them.

Also, I found out that there might be a problem with functions.php, so I entered various internet codes, but they didn't work.

functions.php

<?PHP
add_theme_support('post-thumbnails');
?>

I disable the plug-in, but I still can't see the items in the eye-catch image.
What on earth should I do?