You can generate this message (and you probably don't want to) when using the Edit Cut Profile tool on a Floor slab which has been altered using the Shape Editing tools.
The cause of this message is sketching the Cut Profile at the intersection of the slope condition of the slab as shown in this image. To get what you want, almost, and not get the error message, sketch the profile a tiny distance away from the actual intersection.
In this image the Profile was moved 1/64" to the right, no complaint from Revit.
Now let's backup and review a couple things.
First, if you want to be able to alter a floor slab using the "newish" slab shape editing tools you need to make sure you don't use the Structural Foundation Slab tool. The shape editing tools don't work on them. These are the tools that appear on the Options Bar when you select a Floor Slab.
Second, this example uses a floor slab whose surface has been altered to slope down by 4"(100mm). The floor is defined so that the bottom edge is flat and the surface slopes (via the Layer - Variable check box in the Structure Properties of the floor, un-checked the entire slab slopes instead).
May your day be error free!