Hi,
Not sure if you're trying to request the RMA online, but if you're in the U.S., consider checking with the U.S. ASUS support team to request the RMA by chat or phone if necessary and make sure to contact the Open Platform ASUS support team rather than the Systems team if you are calling about components like motherboards or vga cards, and feel free to provide more details about your product.
https://www.asus.com/us/support/callusIf you want me to help check your serial number, you can send me a message at cl-albert, but I can't issue RMAs, so you may end up talking to ASUS support again either way although if I find a problem with your serial number, maybe you can apply for the online RMA.
Edit: Read your post again and wondering if you are mixing up the terms Case number and Serial number since ASUS support would provide you a case number when you contact them and you normally need to or want to provide the serial number from your product when you talk to support.