I keep Getting a wp-login error

Hi

I created a twentysixteen child theme but now I can no longer edit my site. Everytime i try to login to my admin through https://www.beverlyhillsmdliftandfirm.com/wp-admin/

I get this message:

Warning: Cannot modify header information – headers already sent by (output started at /home/flawley4/public_html/beverlyhillsmdliftandfirm/wp-content/themes/Twentysixteen-child/functions.php:9) in /home/flawley4/public_html/beverlyhillsmdliftandfirm/wp-includes/pluggable.php on line 1171

I am no longer able to edit my site……..I would have to say im a newb so If anyone could give me a “dumb it down” process of what im suppose to do to fix this would be awesomeeee!!!!