/wp-content/themes/atahualpa364/functions/bfa_theme_options.php on line 7

$wordpress_base = home_url();

I'm using WordPress 2.8.6.

You need to upgrade wordpress. Lots of changes were required by wordpress and so if you want to go to 3.6, you must upgrade wordpress

Thanks,

What version of Atahualpa can I use for WP 2.8.6 and above?

With all the wordpress security holes, I would make upgrading wordpress your number one priority.

And make sure you have full backups of both database and files. .. preferably 2 backups (as Murphy's law has it.. "1 backup is no backup") before upgrading.