Hit:1 http://security.ubuntu.com/ubuntu xenial-security InRelease
Hit:2 http://ppa.launchpad.net/freetuxtv/freetuxtv-dev/ubuntu xenial InRelease 
Hit:3 http://ro.archive.ubuntu.com/ubuntu xenial InRelease                     
Ign:4 https://deb.opera.com/opera-developer stable InRelease                  
Hit:5 http://ro.archive.ubuntu.com/ubuntu xenial-updates InRelease
Err:6 https://deb.opera.com/opera-developer stable Release                     
  404  Not Found
Hit:7 http://ro.archive.ubuntu.com/ubuntu xenial-backports InRelease           
Reading package lists... Done                       
E: The repository 'https://deb.opera.com/opera-developer stable Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.