1. You can't put php code in text widget. Install some php widgets plugin and follow that advice.
2. Or do like this:
open loop-single.php file from Graphene Theme and copy all content in new file with same name.
save file and upload it to child-theme folder
open file in editor (the one in child-theme folder) and find this:
<?php graphene_post_nav(); ?>
select and cut (ctrl+x), find this part of code in same file:
<?php /*** Display the post author's bio in single-post page if enabled*/
if ( $graphene_settings['show_post_author'] ) :?>
<h4 class="author_h4 vcard"><?php _e( 'About the author', 'graphene' ); ?></h4>...
and paste it before opening php tag to look like this:
<?php graphene_post_nav(); ?>
<?php/** * Display the post author's bio in single-post page if enabled*/
if ( $graphene_settings['show_post_author'] ) :?>
<h4 class="author_h4 vcard"><?php _e( 'About the author', 'graphene' ); ?></h4>...
It's not neat and fancy but will work :)