"The suitcase belongs to the tall passenger."
Translation:Geamantanul este al pasagerei înalte.
I think the problem is that "Valiza" is feminine and "al" is masculine. If you use "Geamantanul", then you need "al" in the second part of the sentence, but if you use "Valiza", then you need "a" in the second part of the sentence.
(I just got this wrong twice myself and finally figured out that that's probably my error.)
EDIT: And now I've confirmed that "valiza este a pasagerului înalt" is accepted.
No it is not obvious in the English language. If you want to point out the gender of the passenger you would likely talk about a lady passenger or a female passenger.
And since you are learning German too, it is the same here:
Der Koffer des großen Passagiers -masc.
Der Koffer der großen Passagierin -fem.