I am building an EBike. The motor manufacturer said I must replace the existing cassette with a freewheel. Why do I need to do this?
That does not make much sense.
A freehub/cassette combination has the same function as a freewheel. You cannot replace a cassette with a freewheel unless you replace the wheel hub as well.
The only reason I can think of is that the manufacturer thinks freewheels are stronger and can resist the extra torque of the motor better.