Repository to Ubuntu doesn't exist

Clicking the ppa link you gave goes right to (see attached)
However I notice that Thunderbird (my mailer) display an advertisement to the mail from the forum saying "perhaps spam".

Perhaps there is a similar advertisement ? Or the problem is momentaneous ?
Could you try again and send a screen snapshot ?

How did you try to add it? Did you try running these commands from the terminal?

sudo add-apt-repository ppa:mscore-ubuntu/mscore-stable
sudo apt-get update
sudo apt-get install musescore

Notice the PPA link is slightly different to the one you gave in the original post.
Also see this webpage: https://launchpad.net/~mscore-ubuntu/+archive/ubuntu/mscore-stable

I see what the problem is. Your Lubuntu is based on Ubuntu 15.10 Wily Werewolf (released just last month). There is currently no PPA available for 15.10 because the latest version of MuseScore (2.0.2) is already available via the official repositories.

The purpose of the Launchpad PPA is for making the latest version of MuseScore available on older Ubuntu releases. When the next version of MuseScore is released I will create a package for Wily in the Launchpad PPA.

> "What if (whatever reason) I have to re-installed the program?"

Since it's in the official repository you don't need to add any PPAs. All you need to do is:

sudo apt-get update
sudo apt-get install MuseScore