Trouble setting up esp8266 hardware

I am running the Arduino IDE on an Ubuntu subsystem and have successfully gotten it working with an Arduino Uno (uploading the sketch to the board and everything, totally works) and I am now trying to set it up with an esp8266,

I have followed this instruction exactly:
https://arduino-esp8266.readthedocs.io/en/latest/installing.html#instructions-other-os

and I am using the explanations of commands from here:

but I just can't get this working :frowning:

When I run this command:
arduino --upload blinky.ino --board esp8266com:avr:ESP8266_GENERIC --port /dev/ttyS6 --verbose-upload

it returns this error:
Error: esp8266com: Unknown package

Help would be greatly appreciated, thanks!

esp8266com:esp8266:ESP8266_GENERIC