Fix 'E: Unable to locate package' Debian

Demonstrated on Parrot

Either the the package doesn't exist or something is wrong with your repositories. Here is how you fix it

go to:
/etc/apt/sources.list.d

open parrot.list as root.

c & p this into the file:

deb http://deb.parrotsec.org/parrot stable main contrib non-free
#deb-src http://deb.parrotsec.org/parrot stable main contrib non-free

now, run: apt update