Register widget area (sidebar) in WordPress template functions.php file

functions.php

/**
 * Register widget area.
 *
 * @link https://developer.wordpress.org/themes/functionality/sidebars/#registering-a-sidebar
 */
function [name_of_theme]_widgets_init()
{
    register_sidebar(array(
        'name' => esc_html__('Name of widget area', '[name_of_theme]'),
        'id' => '[widget_area_unique_name]', //OnLy LoWeRcAsE!!!!!!!!!!!!!!!!!! else not save widgets on area
        'description' => esc_html__('Description', '[name_of_theme]'),
        'before_widget' => '<section id="%1$s" class="widget %2$s">',
        'after_widget' => '</section>',
        'before_title' => '<h2 class="widget-title">',
        'after_title' => '</h2>',
    ));
}