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:
- Setting the edge you want the Slab Edge to go on to be ‘flat’ by temporarily point editing the slab.
- Then place the Slab Edge (Revit will allow you to, now that the edge is flat).
- 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…