Ashley J. Dearborn had been looking forward to her second dose of the Pfizer vaccine, which was scheduled weeks ago and supposed to take place Tuesday afternoon at a Gold Coast Walgreens. But she awoke to an unpleasant surprise Tuesday morning: an email from Walgreens canceling her appointment. She called the store and learned it only had the Moderna vaccine, not Pfizer, which she needed to match her first dose, she said.