Many WordPress themes allow for featured images, but there are times when the image just won’t show up on the front end, even if you’ve set it in the editor.
This often occurs when the theme doesn’t have the featured image function included in its template files.

Add the below code into your theme’s functions.php file (preferably in a child theme):

function yourtheme_support_feature_img() {
    add_theme_support('post-thumbnails');

    // Optional: set default image sizes
    set_post_thumbnail_size(800, 600, true);
}
add_action('after_setup_theme', 'yourtheme_support_feature_img');