SAN JUAN, Puerto Rico—More than a million customers in Puerto Rico remained without electricity on Thursday after a major power outage hit the U.S. territory Wednesday night, forcing it to cancel classes and shutter government offices. The blackout also left some 160,000 customers without water and snarled traffic across the island of 3.2 million people, where the roar of generators and smell of diesel filled the air. Those who could not afford generators and have medical conditions such as diabetes, which depends on refrigerated insulin, worried about how much longer they’d be without power. Long lines formed at some gas stations as people sought fuel for generators. Others tried to charge their cellphones at businesses in scenes reminiscent of the aftermath of Hurricane Maria, which struck as a Category 4 storm in 2017. Officials in at least one city distributed food to hundreds of elderly people as well as ice …