"Create 3D Slope To Target" will not work

Hello,

I am trying to create a pond with a constant elevation bottom and with 100% side slopes underneath an existing surface. I have been told to do this using "Create Element from 3D Slopes" underneath the Model Detailing Toolbar.

It asks for a reference element (the pond bottom) and for a reference surface, which I give it. Then it asks for "Cut Fill Option", which I tell it Cut Fill. The problem is that upon data pointing to tell it this it asks for side, which is set to left by default, and then immediately (like maybe in a quarter of a second) cuts off and cancels the 3D element tool without creating any feature. I have no idea why it's doing this and it is very frustrating.

Does anybody have any idea what might be going on?

Thank you.