Only the provided repository version of nvtop works with Intel Arc, you cant get it straight from APT (which is what I’m guessing you’re using since I had the same problem), those ones don’t support it.
Either build it from source, or, for Ubuntu >=20.04:
sudo add-apt-repository ppa:quentiumyt/nvtop
sudo apt install nvtop
If I remember correctly, Fedora’s nvtop works with Intel Arc.