OK. Most rear flip/flop hubs come with axles long enough to fit up to 130mm dropouts. Typically, they come spaced for 120mm dropouts, and you add spacers on each side of the hub on the axle to space them out to up to 130mm. The problem is that your dropouts are spaced at 135mm, so you need a longer axle. So either you need to find a wheel with a long enough axle or you need to replace the axle with a longer one. The alternative is to cold set your frame by 5mm, so that it becomes 130mm dropout spacing. That is 2.5mm per side, which is not a lot for a steel frame. You can do it or a shop can do it.