Pandas 1.3 was released today. There are changes that break ActivitySim. At least this line no longer works: https://github.com/ActivitySim/activitysim/blob/a57efa1418320cb88d72ad5f413aaa45131183a4/activitysim/core/simulate.py#L232 As an interim solution, use the latest flavor of pandas 1.2.