Failed Apple watch factory reset

My apple watch is locked. I know the passcode but I get an error message to reset it. It had been paired to an old iPhone which I no longer have. I cannot pair it with my new phone. I cannot access any general on the phone app. I cannot do anything! Why does Apple do this!!! Help

I have tried to reset by charging it and pressing the power button to reset but it still ends up not resetting please what could this be. Will you be needing the serial number because I have tried this so many times. thanks