I want to expand this shape to curve the corners, but it won't let me.

I have created a shape by creating a square and using inverted round 'corners' to create a four- pointed star shape. I have left the points of the star slighly squared still. My next step is to round the corners at each point, but illustrator won't let me. I assumed I'd need to expand the shape to make it a path, but that does absolutely nothing. I have also tried using the pathfinding tool, compound shape, direct selection, etc. but nothing allows me to edit it. See screenshot showing the transform options I have.

TIA!