"Omar has a garage."
Translation:عِنْد عُمَر كَراج.
You can say that.
Personally, though, I would use لدى (ladá). Notice that (3inda) and (ladá) are both correct in standard Arabic even though some linguists do make a difference between the two, but generally speaking for the mainstream Arabic, they are both correct. It's just the way it sounds for a native speaker like me which makes me prefer (ladá) rather than (3ind).
This said, the word كراج is completely non-Arabic and colloquial. It should be مرآب (mir'áb) but anyway Duolingo is doing a "traveler's Arabic" where you can approach people with common knowledge, and this word, garage, is widely used in various dialects.