E: Unable to locate package mosquitto [closed]

Use apt-file or as alternative the online search to search for package names.

You will find that mosquitto is indeed the correct Debian package name to install the server part. The command mosquitto_sub is provided by package mosquitto-clients.

However, the sources.list provided indicates that your installation is not based on Debian but based on Raspberry Pi OS (previously called Raspbian) in version buster. The repository contains the packages as well.

Please check carefully your file sources.list. It seems that the repository is commented out. Remove the # in the first line eventually? The file should look similar as this:

deb http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
deb http://archive.raspberrypi.org/debian/ buster main

# Uncomment line below then 'apt-get update' to enable 'apt-get source'
#deb-src http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi