Can you enter terminal here:
https://forum.qubes-os.org/uploads/db3820/original/2X/5/5b696fe0b5f65457fca3c5e00433f7e251a42d74.jpeg
And check if your USB drive with detached header is present?
Maybe it just has a different letter like /dev/sdd instead of /dev/sde?
Also I’ve just skimmed his guide:
Qubes OS Installation - Detached encrypted boot and header
But I think this:
echo "luks-$uuidR /dev/nvme0n1 /etc/keys/root.key luks,discard,key-slot=1,header=/dev/sda3
luks-$uuidB UUID=$uuidB /etc/keys/boot.key luks,key-slot=1" > $initdir/etc/crypttab
Should be changed from specifying drives by non-static name like /dev/sda to specifying drives by UUID like /dev/disk/by-partuuid/12345678-1234-….
And this one as well:
echo "/dev/nvme0n1 $uuidR
/dev/disk/by-uuid/$uuidB $uuidB" > "{initdir}/etc/block_uuid.map"