Starting October 1, 2020, California residents must use REAL ID cards to board domestic flights in the United States. If you cannot provide a REAL ID card at the TSA checkpoint, you will have to use other valid identification documentation such as a valid U.S Passport or Permanent Resident Card (Green Card).
For more information on California Requirements, please visit: https://www.dmv.ca.gov/portal/dmv/detail/realid/checklist.
For information regarding other states, please visit: https://www.dhs.gov/real-id.