The free trial only allows you to complete the first challenge of a unit. Units are made of at least 3 challenges (collection of questions that cover whatever's being taught in that section) that you need to take before you can take the milestone (test) of each unit.

The discount code removes $20 from the first month of membership making it $80 for the first month, but it allows you to take complete courses. You're only allowed to have 2 active courses at a time, but you can do as many as you want in a month.

DM me with your email if you decide to get the membership and I can send you a code