Yes, actually it is a "fault" of Duolingo. So here is what I looked up in the Danish diccionary (De Danske Ordbog):
Official version: Singular, undefined: en tallerken Singular, defined: tallerkenen Plural, undefined: tallerkener Plural, defined: tallerkenerne
But according to the diccionary, there is an inofficial version which you are also allowed to use. It goes like this: Singular, undefined: en tallerken Singular, defined: tallerknen Plural, undefined: tallerkner Plural, defined: tallerknerne
As you can see, Duolingo used the inofficial version in this section which is not wrong because you are allowed to use it. But in my opinion you should always use the official version when there is one.
Sorry if I did any mistakes concerning the English language.
It is. English is the only language I know which uses a capital letter for the first person (I). It is much more common to capitalise the courtesy forms (English "you" when addressing a stranger): it is done in Spanish (I see you're studying Spanish so you will find Usted (formal you) with capital 'U' but also in Italian and German. None of these languages capitalise the first person.