question

Hey @Rupesh B, after running your model, I found this to be your problem. The error messages were pointing to your Tab Welder_C, so I watched it until right before it threw the error. What happens is that the operator would take a flowitem from the processor before it was finished (the blue one in this gif). If you look at the state of the processor, it is still "processing".

I checked your Process Flow and there's a Wait for Event activity that triggers when a member of the Group("C_LineTab&FlapperGroup") is finished processing and ready to send their flowitem to port. Coincidently, another processor in the group (Tab Welder_D) finished an item as the blue flowitem was starting, but since it is part of the group, it triggered both processors to take off their flowitems.

You may want to change your Wait for Event activity to have the Release Item from the event match the item on the token.