Your $loopimage field in your spreadsheet only has the filename and not the file path, but wp_insert_attachment needs the full pathname in the 2nd argument.
Additionally though, if you have uploaded all of these images using the Media Manager interface then they will already be stored as attachments. You’d be better off using the file path and file name to find the existing attachment ID and then setting that as the featured image of your post.
You can add the current upload path to a filename like this:
$upload_dir = wp_upload_dir();
$filename = $upload_dir[path] . ‘\’ . $loopimage;
// I see you are on Windows, so I’ve used a backslash
But if you are organising your uploads by date, the images you want may not be in this folder.