The reason is that we may not have added "shall/will" everywhere.
In this section of the course, you are learning the near future and near/recent past. We show you the constructions in French and their closest translations so that you may memorize how to use them.
Everyone will have to make an effort: we are going to add "shall/will" everywhere or you will try to remember that all English translations have to translate back to the French original sentence exactly.
I was wondering why "I'm going to enroll myself tomorrow" was not accepted. Seems that sentence and "I'm going to enroll tomorrow" are similar with the difference being the latter sentence has the word "myself" as understood. In English, simply saying "I'm going to enroll tomorrow" leaves open a number of unstated objects of enrollment, e.g. my children, my pet in an obedience class, as well as myself in something. In the French sentence, there is not understood as the construct "m'inscrire" implies that I am enrolling myself.