'Mohamed' is not the only valid English spelling of the Arabic name "مُحَمَّد". 'Muhammad' is another valid and common spelling. In fact 'Muhammad' is phonetically more accurate. But currently Duolingo is accepting only 'Mohamed' as valid spelling.
Spell check called him Muhammad, not me. But it got marked incorrect. Couldn't I at least spend some of my lingos to have my spelling of Muhammad accepted?
the only difference between your answer and mine is that I spelled Muhammad correctly (there is a shaddah over the MIM, so it should be doubled in a transliteration.