I’m getting the following error: Fatal error: Call to undefined function get_post_thumbnail_id()

This error means that your theme doesn’t support thumbnails, in order to add this option please find the file ‘functions.php’ in your theme and add the following strings to this file:

add_action( 'after_setup_theme', 'theme_setup' );
function theme_setup() {
    add_theme_support( 'post-thumbnails' );
}