blog/wp-includes/pluggable.php on line 868

When I try to access the admin page I receive this message.

Warning: Cannot modify header information – headers already sent by (output started at /home/content/e/a/r/earthplanet/html/blog/wp-content/themes/tropicala/functions.php:15) in /home/content/e/a/r/earthplanet/html/blog/wp-includes/pluggable.php on line 868

Can someone point in the right direction? I think it relates to the widgets but I am not sure how to fix it. TIA TOM