Sometimes MicroStation will return an "Element not allowed" message when trying to select an element for clipping a view.
MicroStation cannot use Smart or Feature Solids as Clipping Volumes elements. Simple solids must be use when creating solids that will need to be used as clip volumes elements.
Extruded, simple, non curved profiles generally yield the best results and should work in most cases.
Convert the smart solid to a plain "solid". This should allow you to use your solid as clip volume element
If the solids and/or section are too complex (e.g. curved), it will be required to replace them with extruded simple solids in order to use them as Clip Volume elements