I am new to Arduino. For the project I am trying to do, I want to install the ESP8266 extension. But the additional boards manager URL option is missing from the Arduino IDE. What should I do to add the extension?
You need to download and install a recent version of the Arduino IDE from:
When you install the Arduino IDE using
apt-get you get a very outdated and non-standard version, which does not have the Boards Manager feature. The reason for this is that the license documentation of the Arduino IDE does not currently meet the required standards for the package manager:
the URL is the following and the 1.8.2 IDE accepted it: http://arduino.esp8266.com/stable/package_esp8266com_index.json
Good luck, endre
P.s.: I found the answer/URL here: No Response to AT commands
You stated in the comments that you are using version 1.0.5. This is a very old version. Go through you computer and make sure you uninstalled any old versions, then try to reinstall the ide.
Make sure you download from arduino.cc, the official location to download the software.