A. Ensure that the USB device is properly connected to your iPad. Disconnect and reconnect the USB device, and check if the app recognizes it. If it still doesn't work, try using a different USB port or cable to rule out hardware issues.
B. Check if the USB device is formatted in a compatible file system (like FAT32 or exFAT). If it is formatted in a different file system, you may need to reformat it using a computer to a compatible format before it can be recognized by the app.