The full quotation is as follows:
You are beautiful, but you are empty... One could not die for you.
As with understanding any quotation, it's always important to know the precise context in which it was made. The above remarks are addressed by the little prince to a rosebush. On the face of it, these roses are indeed beautiful, as he freely acknowledges. But all their beauty is on the surface; there's no depth to their beauty.
Most of us at some point in our lives have come across people who are indeed physically beautiful yet unbelievably shallow and superficial. That's what the little prince is driving at here. He compares his own rose to the rosebush that he's now looking at and finds the rosebush wanting.
The little prince had established a loving connection with his rose, watering her, putting her under a glass globe, sheltering her behind a screen, killing the caterpillars that threatened to destroy her, and generally taking very good care of her. But the little prince doesn't have such a connection with this rosebush. These roses are certainly beautiful, but they're empty, devoid of any significance for his life. As such, he cannot imagine sacrificing his life for them as he would do with his own rose, the very special rose that he's grown, nurtured, and loved.