Hmmm, I think it's because of this bug. I reinstalled Mine-imator, and the generate walking animation button was working in an new project. Then I tried opening the project I was working on, and the button didn't work anymore. I deleted every custom resource pack, restarted the application, and the button was working again. Then I tried adding a resource pack back again, and the button didn't work again.
TL;DR Using custom resource packs disables the generate walking animation button