Actually it is supposed to be written but knowing Duolingo so far, there are lot of mistakes (specially in the text-to-speech machine). This -un you hear is actually a vowel on its own and given by the sign ـٌ or sometimes as ـُُ. It is called Tanwin, or Nunation in English. Older extinct languages like Akkadian, had something similar but called Mimation (-um). This vowel plays great part in many things but anyway for the time being you can consider it as a sign for indefinite noun (i.e. it is somehow similar to a/an in English). For this reason, you cannot put the (AL) -the definite article in Arabic- with Tanwin in a single word; They don't fit together.
Is the second word "aamraa2" or "imraa2"? It's written like former but pronounced like latter.
imra2ah (the ending changes to -tun because of adding a vowel to ta-marbúta).
The Alif in the beginning is called Hamzat-Wassl and this one has no ء
They should have added Kasrah below Alif but well, the usual with Duolingo.