Your mistake:
Ff is not = umg.
Instead, Ff = uN, where N is the normal force upward.
The normal force is whatever force the floor has to exert (here,
upward) to keep the crate from penetrating it.
But N is not mg here, because the vertical component of P is also
pulling upward.

If the net work done is zero, and the displacement is not zero
(obviously -- it says the crate is being pulled across a floor), then
the net force in the displacement direction must be zero.

--PD