This BuddyPress error message started occurring in WordPress 5.9. Looking at the WP Core code it is looking for the bp-default Theme in the /wp-content/themes/ folder for this Theme. The BP default theme is loaded from this BP folder: /plugins/buddypress/bp-themes/bp-default.
Best solution: Just ignore the error message since things still work correctly. The error message simply means it cannot find the bp-default theme under your /wp-content/themes folder.
Go for it solution: Copy the entire bp-default theme folder from: /plugins/buddypress/bp-themes/bp-default/ to your themes folder: /wp-content/themes/bp-default. Note: I opted for just ignoring the error message and did not test my BP dev site thoroughly to make sure everything is working correctly.
.
