more info at https://lifterlms.com/docs/lifterlms-sidebar-support/

llms-sidebar-support-2.php

<?php // don't add this line to your functions.php file
/**
 * Declare explicit theme support for LifterLMS course and lesson sidebars
 * @return   void
 */
function my_llms_theme_support(){
	add_theme_support( 'lifterlms-sidebars' );
}
add_action( 'after_setup_theme', 'my_llms_theme_support' );

llms-sidebar-support.php

<?php // don't add this line to your functions.php file
/**
 * Display LifterLMS Course and Lesson sidebars
 * on courses and lessons in place of the sidebar returned by
 * this function
 * @param    string     $id    default sidebar id (an empty string)
 * @return   string
 */
function my_llms_sidebar_function( $id ) {

	$my_sidebar_id = 'my-sidebar-id'; // replace this with your theme's sidebar ID

	return $my_sidebar_id;

}
add_filter( 'llms_get_theme_default_sidebar', 'my_llms_sidebar_function' );

semicolon missing in line 13

thanks

Thomas, I suggest you spell out the need for the full words of the # to be in inside the single quotes in
https://gist.github.com/thomasplevy/795fd98f3073e6872fc36f28e30d05ec#file-llms-sidebar-support-php

I just spent 15 minutes trying to figure out why this wasn't working, as I just put '1', instead of 'sidebar-1' in the quotes.
Might help someone in the future.

I suggest updating line 11 to
$my_sidebar_id = 'my-sidebar-id'; // replace this with your theme's sidebar ID eg. 'sidebar-1'