There are three ways to inflect feminine nouns in Norwegian:
a woman - the woman - women - the women
All feminine (indefinite feminine, definite feminine)
ei kvinne - kvinna - kvinner - kvinnene
All masculine (indefinite masculine, definite masculine)
en kvinne - kvinnen - kvinner - kvinnene
Mixed (indefinite masculine, definite feminine)
en kvinne - kvinna - kvinner - kvinnene
The mixed form is suprisingly common, and is taught for many feminine nouns in this course. However, your indefinite feminine, definite masculine is incorrect.
It would definitely be easier but you would also risk confusion when encountering en kvinne and kvinnen for the first time while on a holiday in Norway.
Therefore, both forms are presented to you so your mind can register it in everyday speech regardless which form you eventually end up choosing for yourself.
This is the second time tonight I've been marked wrong for an answer that is exactly the same as the "correct" one - "the woman has water" - except for the capital letter and the period at the end. I hardly ever type either and hardly ever get marked wrong for it, so this isn't consistent! Tried to report it, but again the option for "my answer should be accepted" wasn't available. Grr.
Always try to write sentences correctly (capitalisation and punctuation) to minimise the possibility of Duolingo marking your answers as wrong even though it can recognise typos which aren't grammatical mistakes as well.
Sometimes I don't do it during practicing sessions but I am yet to come across this bug. It also looks like a problem that should be discussed about with developers and not with contributors.