By default selecting a different Sidebar on a “per Posts” basis is not possible. The Single Post Template pulls in the same Sidebar as the Main Blog Template.
You will find the predefined Blog Sidebar in Appearance > Widgets.
If your preference is to have a different Sidebar on your Posts, please follow the steps bellow:
1 – Access your server via FTP
2 – For Karma Theme go to wp-content/themes/karma/page-layouts
– For Sterling Theme go to wp-content/themes/sterling/
3 – For Karma Theme open the Post Template file for the Layout you have chosen( blog-single-layout-left-sidebar.php or blog-single-layout-right-sidebar.php)
– For Sterling Theme open the single.php file
4 – Scroll down and locate this code near the bottom of the file:
<?php dynamic_sidebar("Blog Sidebar"); ?>
and change to:
<?php generated_dynamic_sidebar(); ?>
5 – Then select a Sidebar from the Sidebar selection dropdown for every Post in Post Edit.
The downside is that you will now need to manually select a Sidebar for each Post you create.
Note that you can create an unlimited number of Sidebars via Appearance > Sidebars.
IMPORTANT: In newer versions of the Themes, the Sidebar selection in Post Edit may not be available.
To fix, access your server via FTP and locate wp-content/themes/(Theme)/framework/extended/
Open multiple_sidebars.php find the following at line 61
add_meta_box( 'truethemes_sidebar_generator', __( 'Custom Sidebar', 'truethemes_localize' ), array( &$this,'edit_form'), 'page', 'side', 'low' );
And add the following just bellow it.
add_meta_box( 'truethemes_sidebar_generator', __( 'Custom Sidebar', 'truethemes_localize' ), array( &$this,'edit_form'), 'post', 'side', 'low' );
Now the Sidebar selection dropdown will show in Post Edit.