Change default sidebar position

Please use the Preset settings to change the default sidebar option selected ( right ) in the post/page edit screen. You can configure the sidebar, header, footer, etc on the existing page and save it as the default preset so that when you add a new page it will apply these preset settings.

Please refer to the detailed documentation here.

Old method
Add this Code in your child theme’s functions.php. It will only apply to new pages/posts, pages posts with already selected sidebar will not affect this. If it does not work, please try adjusting the hook priority

add_action( 'admin_init', 'dt_change_default_sidebar', 30);