If you want to place an elevation datum on a sloped floor or roof, you may have noted that the tool won't work unless you place it along the edge of the floor or roof object.
If you want to place the elevation datum elsewhere, try this workaround: create a new generic model floor hosted or roof hosted family, depending on which object you're wanting to place the datum on. Create a 1" x 1" x 1" void in the family and use the cut geometry tool within the family to ensure that the void is cutting the host.
Load the family into the project and place it where you want to have the datum. Zoom in close to the "nick" in the object you've placed, and the elevation datum tool will now work.