Distance field shadows are only supported on movable lights.
If the directional light is switched from stationary to static, the Light Source Angle Input option is disabled in the details pane.
1. Open Editor (any project, default example map)
2. Select Directional light in Scene Outliner
3. In Details pane, locate Light Source Angle in the Distance Field Shadows submenu.
Head over to the existing Questions & Answers thread and let us know what's up.