Make sure you have a file named opera.list (or similar) in /etc/apt/sources.list.d and that this file contains this (or similar text)
deb http://deb.opera.com/opera/ stable non-free
If it does not, reinstall opera from its deb package here. It is the easiest option you have,instead of editing the file.
Also, please make sure you are on the 64bit version of ubuntu 16.04. You can check that with
uname -m
and it should returb "x86_64".
p.s. I say "or similar" above because I have renamed and edited my files here, e.g. opera-stable.list has become opera.list, so I no longer know their original names.