Duolingo sometimes makes automatic contractions and expansions with certain words (mostly with personal pronouns). It's often helpful, so you don't have to enter both "I'm" and "I am" into the answer set, but sometimes makes for awkward sentences.
In the uk a bun is a sweet thing. We would call it a bread roll or a bap.
Is a bolle the same as a roll? The translation only says 'bun'. We (US English) would use 'bun' for more of sweet roll type of thing, I think. Like cinnamon buns. Caramel buns. etc.