WordPress Header information – headers already sent by

(Sorry this is in english, Im the designer of the theme and I’m trying to set it up on her site, I have done everything to make it work but I just can’t figure it out)

Hello,
Im trying to active a new theme on WordPress but it is giving this error:

Header information – headers already sent by (output started at /data/web/virtuals/240712/virtual/www/domains/avril-lavigne.cz/wp-content/themes/AvrilLavigneWP/functions.php:16) in /data/web/virtuals/240712/virtual/www/domains/avril-lavigne.cz/wp-includes/functions.php on line 12057

Warning: Cannot modify header information – headers already sent by (output started at /data/web/virtuals/240712/virtual/www/domains/avril-lavigne.cz/wp-content/themes/AvrilLavigneWP/functions.php:16) in /data/web/virtuals/240712/virtual/www/domains/avril-lavigne.cz/wp-admin/includes/misc.php on line 1252

Warning: Cannot modify header information – headers already sent by (output started at /data/web/virtuals/240712/virtual/www/domains/avril-lavigne.cz/wp-content/themes/AvrilLavigneWP/functions.php:16) in /data/web/virtuals/240712/virtual/www/domains/avril-lavigne.cz/wp-admin/admin-header.php on line 9

Its not a plugin causing this, or a theme because that is already checked.
What can I do to fix it, or can someone help with this?

Thank you so much!