Hello, I hope the following is of assistance.
As the DPF has been removed when the engine management monitors the DPF for a restriction due to soot accumulation it can detect that there is a discrepancy due to the pressure readings before and after where the DPF was.
If the DPF is removed then the engine management needs reprogramming to take this into account otherwise the fault you describe occurs. There are companies advertising on the internet that can do this but you would need to deal with them directly.
With the DPF removed the MIL will illuminate, if you clear the codes for the DPF fault it may go off for a short time but will quickly illuminate again.
I hope this clarifies things for you but if I can assist further let me know and I'll reply as soon as I'm able to.