"I learn kanji."
Why not use 習う(ならう)? I always have heard 覚える(おぼえる) translated as "remember", never "learn".
習う implies you're being taught by someone else. 学ぶ or 勉強する are closer to "to study" (something).
覚える also means "to memorize", which is what you're doing when you learn kanji or vocabulary.
Then it should really be "I memorize kanji". None of the word choices provided suggest "learn".
If you're learning by yourself, I'd say 勉強する(benkyō suru) would have been the proper verb.
Now the closest translation I'd come to is "I am memorizing kanji" or "I am committing kanji to memory", which is a whole different thing from learning.
"to learn" is a valid translation of 覚える, but I agree that 習う should also be accepted. I've reported it (Dec. 1).
I don't use the word bank, but sometimes I check the "hover hints" before translating, to make sure my word choice is compatible with what duolingo seems to want. But in this case, the hint for "learn" does not include 覚える, and yet 習う (which is there in the hints) is not accepted. Sigh.
Wiktionary: 覚える (transitive, ichidan conjugation, hiragana おぼえる, rōmaji oboeru) 1. to recall, to remember 2. to learn 3. to memorise 4. to feel (an emotion or sense)
See the discussion underneath Risu_kun's comment for more in-depth explanations.