Translation:Someday you will have to sell this car.
Duo claims I have an extra space in "Some day you will have to sell this car."
However, some day is perfect UK English, whereas someday certainly isn't.
They're both correct English, but the difference between the two is subtle.
Thanks for the insight into US English (grammarly.com appears to be a US site which explains US English only). I hadn't realised that some day can be good US English, too.
In the UK we still use some day for both purposes (i.e. as a "compound adverb" and as an adjective+noun), whilst someday remains a spelling mistake. The distinction is deduced from the context:
"Some day, my prince will come..." (future event at an unspecified time);
I've a doctor’s appointment some day next month. (future event within a specified ~30-day period).
However, someday is an adverb so useful that I expect it to enter UK English within the next few decades.
Facit for Duolingo: Please respect two important variants of English by including some day in your vocabulary.
Is " Kiedyś będziesz musieć sprzedać ten samochód. " also correct? Or has "musieć" be in past?