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.
https://guide.the7.io/user-guide/general-guidelines/default-post-settings/

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);