> Could you please tell me the reason why I should move the spatial warping
> step before motion correction?
It most likely won't make much difference either way. There might have a couple tiny advantages with earlier warping, but my main reasoning is this: it would be better to do spatial warping, linear or nonlinear, at earlier stage with the original EPI signal so that you will not have to warp something nonlinear, for example, statistical values such as t, F, or r (correlation coefficient).
Gang