2 things to take into account
a) Opera's package name now is "opera-stable", so you have to use "apt-get install opera-stable" to install it. Using "apt-get install opera" will try to install the old presto opera 12.16, which is obsolete for a long time now.
b) The "this file is not supported" means you tried to install the amd64 package on a 32bit system. If you really are on 32bits, the above will install opera 45.x. which is the last 32bit version of opera.