The package manager tries to fetch multiverse repositories, which do not
exist in Trisquel. Remove the "multiverse" from /etc/apt/sources.list with
your favorite editor (below, with GEdit) and administrator's privileges:
$ gksu gedit /etc/apt/sources.list
In the end, this file should contain the three first or all four lines below
(but you may have chosen another mirror, the URL):
deb http://mirror.fsf.org/trisquel/ dagda main
deb http://archive.trisquel.info/trisquel/ dagda-security main
deb http://mirror.fsf.org/trisquel/ dagda-updates main
deb http://mirror.fsf.org/trisquel/ dagda-backports main
You will then be able to update the packages.
