Does Evards Black Tentacles affect the 5 foot cubes above its area?

So I’m playing in a tomb of annihilation game and there were some succubi that we were fighting. We had information ahead of time that a lot of them were grouped up together talking to someone and so I decided it would be a good time to whip out my new spell Evard’s Black Tentacles. Now the problem is that the Dm said that the creatures are flying creatures and are unaffected.

My problem is this, when the spell designates that the 20 foot square area is the area of the spell, does that actually mean hostile enemies have to be physically touching the ground or does the spell reasonably affect the 5 foot cubes that the 20 foot square has above them.