The functions.php file is the heart of WordPress themes. Reason for that is, this file contain various functions to make your theme work on the way you want it.
Sometimes you or someone else can forget to write some basic functions for your theme.
One of those basic WordPress function is adding support for featured images.
But no worries, one short and simple line in your functions.php file can fix that.
By calling WordPress default function add_support() in your functions.php file, and pass parameter post-thumbnails in this function, you can fix this.
Or just copy and paste following code:
Custom Function Function.php WordPress WordPress
/* * Enable featured image support for WordPress theme */ add_theme_support( 'post-thumbnails' );