My Reply : Chickens quite like dark secluded areas for their nesting areas so not having a window is not necessarily a problem. A lot of coops (especially small ones) don’t have any windows. With large walk-in coops in particular, windows can be quite nice to allow some extra light and air in whenever the weather is good and can also make it a bit more pleasant for you when working in there.
Either way, there should be ventilation in the coop (but no drafts). If there isn’t already ventilation provided in the coop then cutting holes or small window shapes into the walls of the coop can be one way of providing some (and would also allow a little light in). Small holed hardware cloth nailed to the inside of the coop to cover them should prevent anything using the ventilation holes as an entrance. Overall, light is important for your chickens so if it is a larger coop and they are in it a lot it would be better for them if it did have some extra natural light. If they are out and about for most of the day and only in there when laying an egg or roosting, having a window won’t really make too much difference to them (and they probably will be getting some light in from their entrance and the bits of ventilation etc.)