I understand that you are trying to produce an amplitude modulated signal using H-bridge-inverter.
The connections in your model run the IGBT/Diode and IGBT/Diode3 in one cycle which creates a short circuit. If the expected output is an AM signal, try changing the connections from the NOT gate. This can be done by interchanging the connections of either of the pairs of IGBT/Diodes.
Hope this helps!