The Dune series was written by Frank Herbert. If there are new novels in the series added later by other authors as part of an official continuation, I'm not sure who the author of the very last one would be without further research.
Frank Herbert is the original author of the Dune series. But since his passing, there have been other authors who have contributed to the series. To determine the author of the last Dune novel, one would need to look at the most recent publication in the series and check the by - line. It could be one of the authors who have been authorized to continue Herbert's work, like Brian Herbert and Kevin J. Anderson perhaps.
As of now, the main Dune novels were penned by Frank Herbert. But there are also subsequent novels in the Dune universe that are co - written by others. To find out the exact author of the last Dune novel, it's best to check the official sources such as the publisher's website or the book itself. It might be that a new author has taken on the task of continuing the story in a way that stays true to Frank Herbert's vision, but without that research, it's hard to say for sure.