Some SanDisk CFexpress card readers bundled with C500 MarkII kits have been experiencing issues properly seating cards, making transferring data impossible. Canon and SanDisk are aware of this problem and are working towards a solution.
Read this notice for more information from Canon
Read this notice for more information from SanDisk
If you are experiencing issues with your own card reader contact SanDisk customer support
Wrapping rubber bands around the reader to hold the card in the reader has been found to work in a pinch.