"You are a smart girl."

Translation: Tu ești o fată deșteaptă.

[OwlLasers]

Why is, "Sunteți o fata dișteaptă," incorrect?

[Luchtmens]

It's correct (apart from the typo "dişteaptă"). If it was rejected it was probably just because you're less likely to use the formal you (sunteţi) when addressing a girl.

[OwlLasers]

Gotcha. I suppose it could happen though. Haha