"Where does your Spanish girlfriend live?"
Translation:¿Dónde vive tu novia española?
It's because "your" means too "tu,tus,su,sus". In this case "Dónde vive "tu/su" novia española?" it means the same thing. What happens is that in Spanish the "tu/your" is used when we are all friends and the "su/your" when he is your boss or is older than you or you do not know him at all yet.
I hope you understand me.
You did not tell us exactly which Spanish sentence was rejected by Duolingo. But you told us that the next word of your Spanish sentence (after dónde) was su. Instead of su, the right word that should follow after "Dónde..." is a conjugated verb form such as vive. (Alternative verbs that might also work instead of vivir are conjugated forms of residir or habitar.)
This explains why your answer is wrong. The second word of your Spanish sentence is not a verb. Subject-Verb inversion is required after the interrogative adverb, "Dónde..."
The particular preposition that the speaker chooses to include in the Spanish sentence will depend on the sentence.
En dónde ...
A dónde ...
De dónde ...
I just presented three different prepositions. There are actually more than three alternatives if we include the alternative of omitting the preposition. I am sorry; but you are going to have to acquire the skills that are necessary for you to choose which alternative or alternatives make sense in each Spanish sentence that you create. You can't always choose the same preposition every time. You can't always choose de if the Spanish sentences are different each time.
¿En dónde trabaja?
― Where do you work?
― Where do you work at?
¿De dónde es tu novia española?
― Where is your Spanish girlfriend from?
Below, the correct Spanish sentence is shown. In comparison with your Spanish sentence, the only difference is one word. The extra word in your Spanish sentence is "de".
the correct Spanish sentence:
¿Dónde vive tu novia española?
― Where does your Spanish girlfriend live?
The word, de, is a common Spanish preposition. Prepositions are often followed by a noun. Or instead, they are followed by both a determiner and a noun.
novia de un español
― bride of a Spaniard
― girlfriend of a Spaniard