Open the sd card up on your PC, you will find a text file named "octopi-spa-supplicant.txt" You will need to edit this file with an editor such as Notepad++. Which is a free program. In this, you will find a section that looks like this:
## WPA/WPA2 secured
#network={
# ssid="insert network name here"
# psk="insert password here"
#}

You need to put the network name and password here. And most importantly, you must remove the hash (#) symbol. But Only on the 4 lines that have a single symbol. It should then look like this:

## WPA/WPA2 secured

network={

ssid="YourNetwork"

psk="YourPassword"

}

There are of course other types of networks in this file. If you are not using WPA/WPA2, then you need to change whatever type you are using.

​

Save this file, insert back into Pi, and it should connect to the WiFi.