Unfortunately, u can't make walk as an animation name. It can't use walk animation for spell casting. The walk animation only works when you're unit is walking.
Only way is to edit the model and change the "walk" in the animation sequence to "spell" or something. But then again, it will remove that animation when he walks since it was changed to spell.
There is a way though but will make things more complicated.
Like order unit to walk to a point after casting and use:
call SetUnitX(udg_unit, GetLocationX(udg_Point))
call SetUnitY(udg_unit, GetLocationY(udg_Point))
To return unit to the same position where it casted thus making him walk without moving on his place for a brief momment. But this could bug depending on your triggers or something.