The real antaras Auto-Tune?
I don’t know why it doesn’t work but it’s not desiged to be used with Audacity. [u]Here[/u] is a list of officially supported hosts for Auto-Tune. (Audacity is listed as “incompatible”).
Most commercial VSTs don’t officially support Audacity… Sometimes they will work, but most people who by expensive plug-ins are not going to use them with a free or inexpensive Audio Editor or DAW…