My disk is /dev/nvme0n1 in this case I checked in lsblk. I did create the entry but is said one already existed but I cant access it.
I didn’t encounter such issue.
You can try to add the EFI boot entry in your BIOS instead of efibootmgr if your BIOS can do it.
How would I do that
It depends on your motherboard BIOS.
Check your motherboard BIOS manual.
Here is an example:
https://techlibrary.hpe.com/docs/iss/proliant-gen10-uefi/s_adding_boot_option.html
I dont think that version of qubes os compatible with this motherboard rather unlikely I can do anything about it within my skill level