sources list içine

###### Ubuntu Main Repos
deb http://de.archive.ubuntu.com/ubuntu/ xenial main
deb-src http://de.archive.ubuntu.com/ubuntu/ xenial main

###### Ubuntu Update Repos
deb http://de.archive.ubuntu.com/ubuntu/ xenial-security main
deb http://de.archive.ubuntu.com/ubuntu/ xenial-updates main
deb-src http://de.archive.ubuntu.com/ubuntu/ xenial-security main
deb-src http://de.archive.ubuntu.com/ubuntu/ xenial-updates main

bunları yazıp kaydedin ardından

sudo apt-get update dedikten sonra düzelmiş olması gerekiyor.