DMF is not selectable if you are using a lens with a built-in Smooth Autofocus Motor (SAM). Since SAM drives cannot disengage automatically on these lenses, the manual focus ring cannot be freely turned and therefore DMF cannot be enabled.
There is, however, one SAM lens that is compatible with DMF: SAL18135. It uses a special gear to allow the manual focus ring to turn even in auto focus (AF) mode for DMF control.
The following SAM lenses are not compatible with DMF: