In a superflat with cheats enabled.

typed into chat - /bonsaitrees generateMissingShapes yes

I am unsure what else I can do to make it possible to have this code work?
I tried changing the shapes and types folders from Read-Only to open, no change.
I tried with and without cheats enabled.
Any advice?

Playing on MultiMC 0.6.0 with a custom modpack

You'll need to be in creative mode.