#[0] enable contrib repo in /etc/apt/sources.list. Here is how it looks:

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deb http://deb.debian.org/debian buster main non-free contrib

deb http://deb.debian.org/debian buster-updates main non-free contrib

deb http://deb.debian.org/debian-security/ buster/updates main non-free contrib

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Then install it:

apt update

apt search nvtop

apt install nvtop