If you have default Qubes OS partitioning then you should have 3 partitions e.g.:
/dev/sda1 - EFI System partition
/dev/sda2 - ext4 /boot partition - this partition in unencrypted by default and should contain your initramfs that grub is looking for
/dev/sda3 - LUKS encrypted LVM partition
So check your /boot partition content and check its UUID, compare it with the UUID and files that grub is looking for.