We already filed our federal tax returns and only need to file California tax returns

Did you do federal using TurboTax?

I had them done in Washington, I think that’s what they used

TT is not licensed for use by paid tax preparers.

We had volunteers from AARP prepare our taxes

Then no, You cannot use TurboTax to file the CA return...try the link to the Franchise tax Board sight I posted below.

You seem unsure of how your federal return was prepared, and it sounds as if someone else prepared it for you, which means you did not prepare it using your own TurboTax account.  The information flows from the federal return to the state return in TurboTax software.  If you cannot use the same account that was used to prepare your federal return to do a state return, then you need to figure out an alternate way to prepare your CA return.  You cannot prepare a "stand alone" state return using TT.  You could get the forms from the state web site and prepare them by hand, or get them done by a local tax preparer.  You will need a copy of your federal return for 2018 either way.

Unless you used the same TurboTax account to file your federal return and it was accepted, you cant use TurboTax to file your CA return alone.  

You may be able to file for free directly from the CA FTB website.

https://www.ftb.ca.gov/online/calfile/index.asp