The SCM is a flat, twisting strap . It originates at the sternum (as a hard tendon) and the clavicle (as a fleshy belly). As it rises to the mastoid process behind the ear, it spirals.
Stop trusting the generic PDF that flattens the human form. Look for resources that show you the twist , the tilt , and the tension between the head and the neck. head+and+neck+anatomy+for+sculptors+pdf+exclusive
Place your hand on your throat and swallow. Feel that bone lift? That movement creates a shadow pattern under the jaw. In your sculpture, even a subtle shadow here suggests a lean, anatomical neck. Ignoring it suggests a neck that has been smoothed over by filler. The SCM is a flat, twisting strap