Qucs não suporta o Ubuntu 15.10.
Além disso, se você quiser instalá-lo na versão anterior, substitua:
sudo add-apt-repository ppa:qucs
Com:
sudo apt-add-repository ppa:qucs/qucs
$ sudo add-apt-repository ppa:qucs
[sudo] password for hsthakar:
Cannot add PPA: 'ppa:~qucs/ubuntu/ppa'.
ERROR: '~qucs' user or team does not exist.
$ sudo apt-get install qucs
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package qucs is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'qucs' has no installation candidate