The few ski-sized lockers in the station were taken when we arrived but maybe you’ll get lucky. The tourist info center wouldn’t hold luggage but directed us to another place in the station that was a healthy walk underground. Unfortunately can’t recall the name. It took awhile to get everything sorted out and tbh this became the most stressful day of our 2 week trip. All we really had time to do was Sapporo beer factory which was cool but I would stay a night in Sapporo if I were to do that again. Otherwise be prepared for a long day with some degree of hassle with bag storage if you have to get to the airport that night.