I get the hovering "You have an extra space" for entering "coffee maker" instead of "coffeemaker". However, checking online, the spaced version seems to be both correct and preferred (26M google hits vs. 5M).
It's the compound words rule.. You always put nouns together in Danish. It's not correct not to.
(Don't trust google on this one, with the extra spacing you will be shown hits that includes other content then just 'kaffemaskine' - also for SEO reasons companies might intentionally split words, even if its wrong.)
In some cases spacing will actually change the meaning of a sentence.
Thanks for the info. However, my remark concerns the English text (required as a translation reply to the Danish text).
Maybe duolingo thinks a coffee maker is the person who uses a coffee machine.