You probably used the wrong gender article -- the word Schmuck is masculine so it is der Schmuck.
If you used das Schmuck or die Schmuck instead, that would be wrong.
Unfortunately, Duolingo is not always very smart about corrections -- in particular, if the article doesn't match the noun, but there is an accepted alternative that does use that article it will suggest that, rather than correcting the article.
I would say "jewellery" in English is uncountable singular. "The jewellery is in the box" could mean one piece, or many pieces. But I wouldn't say 'the jewellery are in the box', or 'the jewelleries are in the box'.
It works similarly in German: Der Schmuck can refer to one piece or many pieces. There's no commonly-used plural form. If you wanted to be specific, you could say "three pieces of jewellery" in English, or drei Stücke Schmuck in German.
What happened is that old English used to have se, seo, þæt as the masculine, feminine, and neuter definite articles, but then later lost grammatical gender and decided to use þæt (now spelled that) as a demonstrative and se (now the) as the definite article.
German has kept all three separate genders, and has also kept four cases, while English lost cases entirely on nouns and has only two cases for pronouns (e.g. he versus him).