I've followed the instructions at https://certbot.eff.org/all-instructions/#debian-8-jessie-nginx but cannot install certbot:
sudo apt-get -t jessie-backports install certbot
Reading package lists... Done E: The value 'jessie-backports' is invalid for APT::Default-Release as such a release is not available in the sources
Hints appreciated
deb http://ftp.debian.org/debian jessie-backports main
. Ranaptitude install python-certbot-apache -t jessie-backports
after adding to /etc/apt/sources.list. GotE: The value 'jessie-backports' is invalid for APT::Default-Release as such a release is not available in the sources
printed twice.