I work for UPS and my manager is instructing us drivers to double park. I refused and was terminated, can he do this?

You cannot be fired in retaliation for refusing to commit an unlawful act, even though employment in California is at-will.  You should consider filing not only for unemployment but also filing a complaint through the California Dept of Industrial Relations and hiring an attorney if the agency cannot help you in a reasonable amount of time.

Specifically, you must file with the Dept of Industrial Relations within six (6) months of the retaliatory act.  Filing a separate court action through the aid of private counsel is a separate right and process than the one filed through the Dept of Industrial Relations.  You don't risk losing one path by following the other.  They can be simultaneously filed under most circumstances or you can probably await the outcome of the agency action before filing in court.