Can I program a set range instead of a set number while building a workout?

You can not program a set range inside the "sets" area of a workout, only on the reps section. You can however make a set recommendation inside of the notes section of each exercise. 

Your client also has the option to add additional sets to an exercise. They can do this by clicking the three dots next to an exercise, and hit the 'Add Extra Set' button (see picture below). They are not able to remove sets that you have prescribed.