Hi Marija,
impact-coding (or target encoding) refers to a preprocessing method where the categorical feature in question is encoded by using the target variable. A little more info here: http://contrib.scikit-learn.org/categorical-encoding/targetencoder.html
As you can see, in the context of clustering (unsupervised learning) where no target variable is known, this preprocessing option is not available in DSS.
I hope this helps!