It can be, as long as the sail is set up in such a way that incoming air is deflected backwards to some degree rather than at a right angle to the fan (such as with a curved sail). Consider a still boat in still air (thus with no net acceleration in any direction to start). You push some air forwards towards the sail, then the sail deflects that stream of air outwards and slightly backwards. This means the combined fan-sail system (the ship taken as a whole) is taking in initially still air, and that air ends up with a net momentum backwards, so there must be push forwards. This is very inefficient compared to doing away with the sail and pointing the fan the other way, so theres no reason to do it in practice, but it doesn’t violate conservation of momentum or anything because the ship isn’t a closed system.