If you do not see the set featured image box beside the post editor in the WordPress admin panel to set the featured image of posts, your solution is here. Just copy/paste this code snippet to the function.php file of your theme.
add_theme_support function in WP will register a feature for the specified theme. Here are a few examples of using the add_theme_support() function in WordPress development.
Learn how to add a new section to the settings page of the WordPress admin panel with examples. In this WordPress tutorial, we’ll introduce the WordPress add_settings_section function and review its usage with examples.
Learn Extending Posts Query in WP, Join Custom Tables and add new fields to Post Items In this advanced WordPress Development tutorial.