Why do I get “PCI device dom0:03_00.0 does not exist” when trying to run any Qubes OS VM? How can I fix it? I am a total beginner in Linux so please don’t make much fun of me 
Probably you attached the device to some VM and the ID was removed later for whatever reason (hardware change or BIOS update).
Remove 03:00.0 from whatever VM that you attached it to. Check the VM settings (devices tab) of your VMs for it.
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Thank you! Everything works fine now