BuddyPress.org

This is a theme issue. The new bp-default has a main content area (where you’re seeing the “not found” message which signifies that you haven’t yet posted any blog entries on the main blog), and one sidebar at the right. You’ll have to create a child theme and register some new sidebars if you want to use the new bp-default and have a similar look to the old bp-sn-parent.

Here’s a discussion on the subject: https://buddypress.org/forums/topic/three-columns