In some cases, when Plank is set to transparent and autohide mode, it is very hard or even impossible to open the Plank preference when those modes are activated. But don’t worry, this simple trick can be used to gain access to Plank Preferences.
Usually, Plank Preferences can be opened via right click on the Plank dock. But, it can also be opened via Terminal. Use the following command to open Plank Preferences.
Hope this trick useful for you who struggling to open Plank Preferences.