Because the verb has a direct object here (the language).
Remember that verb prefixes mark not only the subject, but also the object or absense of one.
bI- indicates "subject = you [one person]; no object" while Da- indicates "subject = you [one person]; object = him/her/it/them".
In this case, the object of Hol "the language", which counts as an "it", so you need the prefix Da-.
If you want to refresh your memory of those two prefixes, have a look at https://www.duolingo.com/skill/kl/Useful-phrases/tips-and-notes .
If you didn't know of the existence of the tips and notes: they're available on the website https://www.duolingo.com/ but not (so far) on any mobile app, at least not for Klingon.
Visit the website in a browser, then click on a lesson unit and then on the lightbulb icon: