Can I file taxes in California for 2021using the online platform?

Online is not possible.

https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/prepare-2020-2019-2018-tax-return/L6De9oWON_US_en_US?uid=ltxnqnet

No you cannot use online for any past year.  Online is only for 2023.  

 

If you prepared your 2021 federal return using TT you can go back and use the CD/download to prepare the state return.  Or you can go on the state's tax web site to get the forms to prepare by hand... Or seek local paid tax help.

 

https://ttlc.intuit.com/questions/2559403-how-do-i-finish-a-prior-year-return-that-i-started-in-turbotax-online

 

https://ttlc.intuit.com/community/state-taxes/help/how-do-i-contact-my-state-department-of-revenue/00/25573