Unable to add board outline to Gerber files

Hello,

I have my schematic and PCB finished, but when I generate the GERBER files there is no board outline. I tried following the guide here: https://www.rs-online.com/designspark/how-can-i-add-a-board-outline-to-my-gerber-plots

but when I create a new layer, renamed as Board Outline, it doesn't show up on the list in the centre of the pop-up window, under the 4 tabs (see attached screenshot). Any ideas how to generate the board outline in the Gerber files?

Thanks.