By default, KlipperScreen does not have special support for USB disks in the way that the Creality modified version does.
Install and set up SPD-usbmount
https://github.com/qwazwsx/SPD-usbmount
Refresh the file listing in KlipperScreen and (ONLY if your flash drive contains .gcode files) you should see a new directory called USB containing the contents of your USB device.