can't login to my admin

[ Moderator note: title adjusted, please refrain from using help, urgent, emergency, asap, etc. in topic titles. ]

Can anyone help me. I’m fairly new to this, so need a little help. Somehow I have done something to get this message when trying to log in.

Warning: Cannot modify header information – headers already sent by (output started at /home2/luisadigi/public_html/sunstatepainting/wp-content/themes/zerif-lite-child/functions.php:18) in /home2/luisadigi/public_html/sunstatepainting/wp-includes/pluggable.php on line 1171

I am using a child theme I created from Zerif-Lite. This is the site I’m working on:
http://sunstatepainting.luisadigirolamo.com

Any help would be appreciated 🙂