Despite the success of the first two movies, it might seem unusual that Dune: Part Three hasn't been announced yet, but there may not be cause for concern. Given that the first two entries in Denis Villeneuve's Dune trilogy have done so well, it would be understandable if the studio immediately greenlit the third movie in the series. However, while that isn't the case just yet, Warner Bros. are holding fire on any further Dune announcements for a good reason. In short, the studio is waiting for confirmation from one key figure that they're ready to continue with the franchise.

https://screenrant.com/dune-3-not-confirmed-greenlit-reason/