To answer your question, it will help to clarify that creatures can pass through the wall. So when the wall is created, and a creature occupies the same space of a section of the wall, they have to make a save. Here is the text of the spell
When the wall appears, each creature within its area must make a Dexterity saving throw. On a failed save, a creature takes 5d8 fire damage, or half as much damage on a successful save.
One side of the wall, selected by you when you cast this spell, deals 5d8 fire damage to each creature that ends its turn within 10 feet of that side or inside the wall. A creature takes the same damage when it enters the wall for the first time on a turn or ends its turn there. The other side of the wall deals no damage.
So the answer is YES, you can cast the spell with a creature occupying a space of the wall, and they'll have to Dex save to avoid some damage. And if they want to pass through the wall itself later on, it's going to cost them some fire damage.