Voltage Clipping with Two Series-Opposing Zener Diodes
The simulation does work correctly. But you cannot see the clipping, because the currents are too high. Consider those Z-Diodes you selected are specified for 1.3W. So they will have a steep differential resistance. And your setup forces 10A through them.

Increase R_V and R_L to reasonable (K) values (I recommend choosing R_L >> R_V) and select your input voltage accordingly. Then your simulation will yield something visible.