Translation:By tomorrow we will have attended all the courses.
The dictionary definitions given are not reliable. I used one of them and was marked wrong. (Instead of "by" I put "for". (Duo's translation is correct. But the so-called dictionary references are misleading. ) This is not proper teaching methodology. It is "testing" without teaching. A failing grade for Duo.
DL needs to be a lot more forgiving with this entire lesson. Future perfect tense is the least used tense in English, even among writers, leaving people very unfamiliar with it. Add to that DL constantly changing perfect tenses into imperfect ones in their own translations, and the amount of grammar Nazi nonsense in this is completely unacceptable.
It is an idiomatic use of "seguire": their "to follow a course" is our "to take a course" or "to attend a course". It's just the word they use. Translating it as "follow" isn't good English.
Lots of examples below: http://context.reverso.net/translation/italian-english/ho+seguito+i+corsi