What's the difference?
Its being used as a predicate adjective (adjectives following the linking verb "to be". Predicate adjectives don't get the "-e" ending per the Duolingo tips and notes.
Nederlands is a 'het' word?
I selected the correct answer and it was rejected
Die richtige Lösung stand nicht zur Auswahl! You could not choose the right word!
"As you can see in the last example, niet comes after the object, unlike geen. If it is used to negate an adjective or adverb, it comes directly before that word."