It seems the Murf AI people removed the Finn voice for some reason. As that was the default in the Apple version, that will cause some errors. I am working on a fix to pick a new default and better handle missing voices from Murf AI. I have requested that Murf put that voice back as I suspect thousands of people were using it. Please watch this thread for updates.
As a workaround for now, you should be able to pick a different voice and it should work.