Fatal Error using Simplecart

Not sure if this is the correct section to ask this or not:

I am trying to create a post for my front page, but when I try to attach images of products for illustration purposes, I get the following error message:

Fatal error: Call to undefined function get_post_thumbnail_id() in /home/content/98/6575898/html/wp-admin/includes/media.php on line 1314

The images work fine for individual products, but not when attaching any image to a post or new page, regardless of source (desktop, iphoto, media). It has worked in the past, so not sure what is different. I use in conjunction with the SimpleCart 2.1 theme.

So far, I have disabled all plugins, but that made no difference. I have re-installed WP 3.1.3 as well as Shopp and SimpleCart to no avail.

Any help is appreciated.

Thanks