[Multisite Content Copier] PHP error when trying to copy specific posts using Multisite Content Co

I am receiving an error message in our php error logs (below) when trying to copy specific posts from one network site to another when selecting copy images. Sometimes a portion of the selected posts will come over, other times none will. If we go back and repeat the process and leave the "copy images" checkbox unchecked, the posts all come over but then the image urls are referencing the source site within the actual html link. If you look in the media library sometimes you see placeholders where the images would be and they look like broken image previews (ie the photo isn't showing). So I imagine it tries to copy the image, can't copy it or copy its metadata and then fails to do the find/replace within the post content since it thinks the image hasn't been copied.

Error from log:
PHP Fatal error: Call to undefined function wp_generate_attachment_metadata() in /home/catapult/public_html/wp-content/plugins/multisite-coontent-copier-post-type.php on line 562