Because your selection is in the category of "Geometry Selection". For your nodal/elemental selection, you could define a Named Selection object with a group of nodes/elements, and apply it to the boundary condition.
Hi I tried that. I created a Named Selection for the boundary nodes (called "Selection" in the picture) but when I try to use that Named Selection it does not appear as an option in the Named Selection pull down...
My model behaves the same way (17.0). There seems to be a restriction in Workbench. Is there a particular
reason you can't use the bottom edge instead of nodes?
It seems that you are trying to apply a boundary condition designed to work on geometry to a FE-entity. When you apply a boundary condition to the FE-entities you will have to use boundary conditions for same.