call to undefined function error?

Hello tried to understand this from reading other posts but i am now confusion myself!

i have uploaded my theme which is in the theme folder but i am getting this error when i view my website;

Fatal error: Call to undefined function get_page_sidebar_settings()

the last part is this;

page.php on line 18

this is the code on line 18:

$page_sidebar = get_page_sidebar_settings($post->ID);

could anyone shed any light on this!