two possible reasons: - negation (not) takes construction with do/did. (almost always). But also: has believed is another verb tense in Italien, ha creduto, instead of credeva ... that's what I believe in this context here.
"Qualcuno" can be "someone", right? Qualche=some, uno=one. Plus the drop down menu (the tips) says "someone" is a viable response. So why is that a wrong answer here?
I am not a native english speaker- why not anybody as there is a negation instead of somebody??
Anybody doesn't sound right in English. As a native English speaker, I can't really tell you why. I just know it sounds very odd. I would say that somebody is more of a "searching for a specific person/group" word and anybody is a "free-for-all/everyone is invited." I'm not sure if this makes sense to you. It's hard to explain. Hopefully someone else will come along to explain it better than me.