The title speaks for itself, I followed the guide (and the key changing that can be found on different guides online), but I can't install ros melodic due to "Unable to locate package" I'll add all the steps:

luca@luca-lx:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.6 LTS
Release:    18.04
Codename:   bionic
luca@luca-lx:~$ sudo sh -c 'echo "deb http://packages.ros.org/ros/ubuntu $(lsb_release -sc) main" > /etc/apt/sources.list.d/ros-latest.list'
[sudo] password for luca: 
luca@luca-lx:~$ sudo rm /etc/apt/sources.list.d/ros-melodic.list 
luca@luca-lx:~$ cat /etc/apt/sources.list.d/ros-latest.list 
deb http://packages.ros.org/ros/ubuntu bionic main
luca@luca-lx:~$ clear
luca@luca-lx:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.6 LTS
Release:    18.04
Codename:   bionic
luca@luca-lx:~$ sudo sh -c 'echo "deb http://packages.ros.org/ros/ubuntu $(lsb_release -sc) main" > /etc/apt/sources.list.d/ros-latest.list'
luca@luca-lx:~$ cat /etc/apt/sources.list.d/ros-latest.list 
deb http://packages.ros.org/ros/ubuntu bionic main
luca@luca-lx:~$ sudo apt-key adv --keyserver 'hkp://keyserver.ubuntu.com:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
Executing: /tmp/apt-key-gpghome.Yptypss1Bp/gpg.1.sh --keyserver hkp://keyserver.ubuntu.com:80 --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
gpg: key F42ED6FBAB17C654: "Open Robotics <info@osrfoundation.org>" 1 signature cleaned
gpg: Total number processed: 1
gpg:     signatures cleaned: 1
luca@luca-lx:~$ sudo apt update
Get:1 http://security.ubuntu.com/ubuntu bionic-security InRelease [88,7 kB]
Hit:2 http://it.archive.ubuntu.com/ubuntu bionic InRelease
Get:3 http://it.archive.ubuntu.com/ubuntu bionic-updates InRelease [88,7 kB]
Get:4 http://it.archive.ubuntu.com/ubuntu bionic-backports InRelease [83,3 kB]                  
Hit:5 http://packages.ros.org/ros/ubuntu bionic InRelease                                                              
Fetched 261 kB in 3s (83,5 kB/s)                                            
Reading package lists... Done
Building dependency tree       
Reading state information... Done
All packages are up to date.
luca@luca-lx:~$ sudo apt install ros-melodic-desktop-full
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package ros-melodic-desktop-full
luca@luca-lx:~$ apt search ros-melodic
Sorting... Done
Full Text Search... Done
luca@luca-lx:~$ 

I found that there are some 'ros-desktop' packages, however I can't install them.

I don't know if it's useful:

luca@luca-lx:/etc/apt/sources.list.d$ apt search ros-desktop
Sorting... Done
Full Text Search... Done
ros-desktop/bionic 1.10 all
  Robot OS desktop metapackage

ros-desktop-dev/bionic 1.10 all
  Robot OS desktop development metapackage

ros-desktop-full/bionic 1.10 all
  Robot OS desktop-full metapackage

ros-desktop-full-dev/bionic 1.10 all
  Robot OS desktop-full development metapackage

ros-desktop-full-lisp-dev/bionic 1.10 all
  LISP Robot OS desktop-full development metapackage

ros-desktop-full-python-dev/bionic 1.10 all
  Python Robot OS desktop-full development metapackage

ros-desktop-lisp-dev/bionic 1.10 all
  LISP Robot OS desktop development metapackage

ros-desktop-python-dev/bionic 1.10 all
  Python Robot OS desktop development metapackage

luca@luca-lx:/etc/apt/sources.list.d$ sudo apt install ros-desktop-full
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 ros-desktop-full : Depends: ros-desktop but it is not going to be installed
                    Depends: ros-perception but it is not going to be installed
                    Depends: ros-simulators but it is not going to be installed
E: Unable to correct problems, you have held broken packages.