Actually, for 2D billboards there are just versions for Standard
, V-Ray or Mental Ray materials. For proper use of 2D billboards the Forest Material
should be used. As in case of Octane Renderer this is not supported there is no material library for it.
Anyway, if you decide to give it a try in Octane renderer, please use the Standard
option - you can find it on the bottom of the Library Browser
's window (please see attached screenshot).
Thanks for reporting us this "bug".