Revit does not want you to place a slab edge on part of a slab that is sloping (because it has been shape-edited).

However, you can ‘trick’ Revit into letting this happen by:

  1. Setting the edge you want the Slab Edge to go on to be ‘flat’ by temporarily point editing the slab.
  2. Then place the Slab Edge (Revit will allow you to, now that the edge is flat).
  3. Adjust the points back to how you want them.

The Slab Edge should happily follow the sloping edge now.

A bit of a workaround, but it does work…