Element not Valid for Tool

When trying to add breaklines to the triangles created by my scaleable terrain model, I go to FEATURE MANAGEMENT - ADD FEATURES, it prompts me to "Locate terrain model to add elements" but when I go to click on the terrainj model it tells me "ELEMENT NOT VALID FOR TOOL, SCALEABLE TERRAIN MODEL".

How do I fix this in order to add my breaklines to this TIN?