some related thread:
https://forum.qubes-os.org/t/install-qube-os-in-virtual-box-wont-work-why/17175
https://forum.qubes-os.org/t/qubes-os-under-virtualbox-for-tests/8187
https://forum.qubes-os.org/t/cant-boot-qubesos-4-2rc1/19086
from the first link:
Don’t know if still usefull for you, as you appear to try to resolve the install on USB in an other thread.