Cannot find the ESP8266 board

I have looked in the IDE for the ESP8266 board but cannot find it and cannot see how to add it.
I am wanting to program a sonoff basic from my Linux Debian lattop and I have install Arduino IDE 2:1.0.5+dfsg2-4.1.
Please tell me what I have missed.

you have to add the link to the ESP8266 in file > preferences

ADD ESP8266

straygrey:
I have install Arduino IDE 2:1.0.5+dfsg2-4.1.

This is your problem. APT has a very outdated version of the Arduino IDE. That version doesn't support installing 3rd party boards in the manner required to use ESP8266. You'll need to use a modern version of the Arduino IDE. Although I believe you can get a modern version from Snap, I would recommend just downloading the Arduino IDE from the website:

Make sure to remove that ancient version you got from the APT repository to avoid confusion.

To be clear, after installing the modern Arduino IDE, you still need to follow the instructions provided by @dave-in-nj.