I am answering my own question.
It seems that when running a "referenced" model in SIMULINK version 2024 it defaults to a "Accelerator" in the "Simulation mode" instead of "normal" which I suspect was the default in 2023. As I an relatively new to SIMULINK and the fact that it compiles in the "accelerator" mode I was getting an error I did not really understand.
Once I switched to "normal" the SIMULATION ran as per normal. Presently, whilst debugging the model, running in "Accelerator" mode makes no real sense. But eventually I suppose I will need it.