This is due to how you burn your ISO to your USB stick. Most graphical tools are useless which is why dd is often recommended when creating an installation media on Linux. However, it seems you got the system to boot so what you can do now is basically to unplug the USB stick for about 30 seconds, plug it back in and wait another 30 seconds then try to mount the "CD" again.
This time, the installer will detect and mount the USB as the CD and successfully install from that. The suggestion was found here for reference: https://null-byte.wonderhowto.com/forum/fix-failed-detect-and-mount-cd-rom-problem-when-installing-kali-linux-0177158/
