I had the same problem and i managed to resolve the issue by using this code.
//insert featured image
function Generate_Featured_Image( $featured_image, $product_id ){
$upload_dir = wp_upload_dir();
$image_data = file_get_contents($featured_image);
//$filename = basename($featured_image);
$filename = basename($image_url, '.jpg') . (string) rand(0, 5000) . '.jpg';
if(wp_mkdir_p($upload_dir['path'])) $file = $upload_dir['path'] . "https://wordpress.stackexchange.com/" . $filename;
else $file = $upload_dir['basedir'] . "https://wordpress.stackexchange.com/" . $filename;
file_put_contents($file, $image_data);
$wp_filetype = wp_check_filetype($filename, null );
$attachment = array(
'post_mime_type' => $wp_filetype['type'],
'post_title' => sanitize_file_name($filename),
'post_content' => '',
'post_status' => 'inherit'
);
$attach_id = wp_insert_attachment( $attachment, $file, $product_id );
require_once(ABSPATH . 'wp-admin/includes/image.php');
$attach_data = wp_generate_attachment_metadata( $attach_id, $file );
$res1= wp_update_attachment_metadata( $attach_id, $attach_data );
$res2= set_post_thumbnail( $product_id, $attach_id );
}
Generate_Featured_Image( $featured_image, $product_id );
what you have to do is to pass parameters upon calling functions.
Code goes to your theme or child-theme’s functions.php file.