I don't understand what is actually happening, but I tried installing Steam, Opera and both times was unsuccesful. I think the program "Ubuntu software" is not working properly. For instance, if I type "Opera" on Ubuntu Software, an option to download and install the browser should show up, but it doesn't.
When I download the programs and try to install them (both opera and steam) via Ubuntu software, the following message appears on the Ubuntu Software window: "This software comes from a third party and could contain non-free components". And although I press the button "Install", nothing happens.
I was able, however, to install VIM, Synaptic, GNU.
Have anyone ever had a problem like this? Can anyone imagine how to solve this?