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I make that same mistake at least twice a day! I wish they had a clearer distinction between translation and dictation exercises. Once you start typing, the instruction disappears. Also I tend to start typing in whichever language my keyboard is set to, without thinking.
It seems to be the way the whole Duolingo system is set up: Have a look at the FAQ page https://www.duolingo.com/comment/16292319 This explains why we don't need to worry about nikkud too much. Basically if it marks you "correct, but with a typo" on some exercises it's because the software is looking for what it thinks is an exact match to its "model answer". I can't do nikkud on my phone either: on a desktop/ laptop with Hebrew input enabled, the nikkud are accessed as [alt]+ number key (MacOS). e.g. (L [alt]+7) for ךָ
The contributors to the Hebrew course must've been tearing their hair out trying to make it work, as the way Hebrew is written doesn't sit well with the way the software is structured. It look like they're doing the best they can so that we still get learn, one way or another.
See the "Welcome to Hebrew" post at the beginning of the course. For a technical explanation... ask someone who knows more about it. For a beginners' version, it's the system of dots and things placed next to /under letters to give a clue as to what vowel-sounds to use. In modern Hebrew they tend not to be used much, as one is supposed to be able to work it out by context, most of the time:-\