Thank you for your help, Magic Banana!
I have accessed the VPN configuration tool now. :)
I am sorry about not reading the output. I am not very good at it.
Here is the output:
cat /etc/apt/sources/list
cat: /etc/apt/sources/list: No such file or directory
Here is the content of /etc/apt/sources.list
# Trisquel repositories for supported software and updates
deb http://us.archive.trisquel.info/trisquel/ brigantia main
deb-src http://us.archive.trisquel.info/trisquel/ brigantia main
deb http://archive.trisquel.info/trisquel/ brigantia-security main
deb-src http://us.archive.trisquel.info/trisquel/ brigantia-security main
deb http://us.archive.trisquel.info/trisquel/ brigantia-updates main
deb-src http://us.archive.trisquel.info/trisquel/ brigantia-updates main
# deb http://ftp.rediris.es/mirror/Trisquel/packages/ brigantia-backports
main
# deb-src http://ftp.rediris.es/mirror/Trisquel/packages/ brigantia-backports
main
