If you were a non-resident of CA for all of 2020, and you were a W-2 employee, and you never physically worked within the state of California the entire year, then your income for that year is not taxable by CA.
CA taxes non-residents only on CA-source income. W-2 income is "sourced" where the work is actually (physically) performed. If you worked solely from a WA location, then your income from that work is Washington-sourced, not California-sourced. If this is the case, you should reply to the notice by explaining that in 2020 you were a non-resident of CA, and that you never worked within the state of CA in that year.
From CA FTB Publication 1031, page 6:
"Wages and salaries have a source where the services
are performed. Neither the location of the employer,
where the payment is issued, nor your location when
you receive payment affect the source of this income."
https://www.ftb.ca.gov/forms/2022/2022-1031-publication.pdf
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
