I modified my theme metabox.php to fit my needs but i get this error when i go to add a new listing :
Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘mytheme_add_box’ not found or invalid function name in E:\xampp\htdocs\wordpress\wp-includes\plugin.php on line 406
I have a online version of this wordpress site but it is not modified if it helps this is the website http://www.dreamsimob.ro
Activate that child theme and try your modifications on that child theme instead. It’s safer that way.
Whatever you do, do not copy the parent theme’s functions.php file into the child theme directory. That just won’t work. Whatever changes you wanted to make usually they can be made by editing copies of the theme files such as header.php or footer.php.
For more specific help then consider contacting the theme author.