Hi Chee,
It’s a good thing that you started a new topic for this problem. However, I posted a comment on this at the topic below:
optiwave.com/forums/topic/error-during-simulation/
I couldn’t find the exact problem with your system, but I used your channel in optisystem sample OFDM implementation and there it worked, at least for a back to back system. Please refer to my answer.
Regards