I downloaded the lastest version of arduino 1-8-5 from arduino.cc to run a linux (ubuntu 16.04) and when I launched it, i got the following (see attachment) but when I select board, I cannot find "board Manager".
Maybe you installed Arduino IDE 1.8.5 but that's not the version running in your screenshot. The version running in your screenshot is this Arduino 2:1.0.5:dsfg2-4 garbage that you get from the apt-get arduino command. Delete that version and use the modern IDE version downloaded from: