Same story. Something seems out of sync. When I enable voicemail for an extension in the GUI it shows as novm in the extensions config. When I disable it it'll start showing up properly in extensions_additional.conf but play the "I'm sorry" message.
Just created a new extension, before even saving it I set VM to be on, same story. exten-vm,novm. Turn VM off and it sets up the config properly but obviously disables the rest of the needed VM config elsewhere.
I think something is broken with my install. I might spin up a new FreePBX VM and see if I can re-create the issue on a fresh install. As they say, 50th times a charm 
Thanks for the help!