A previosly purchase product, CA turbotax 2018 does appear in the downloads

[HelenC12]

The most common reason for not finding returns is signing in with the wrong User ID, which can happen if you have multiple accounts. It’s best to use the same account every year so all of your tax returns will be stored in one place.

If you need to find an account, our account recovery tool can help you find your User ID or password.

Other common situations include:

I used a CD/Download version of TurboTax instead of TurboTax Online

I filed my return with H&R Block, TaxACT, or somebody else

I filed another return using the same login

Source: TurboTax FAQ

[timhwithers]

HelenC12,

It appears you misread/misunderstood the question.

The question asks about a 2018 CA download, not the tax return. 

So, how can we download a missing 2018 State (CA) software?

[VolvoGirl]

Access your downloads here

https://shop.turbotax.intuit.com/commerce/account/secure/login.jsp?pagecontext=download

How to download your software purchase

https://ttlc.intuit.com/community/installing/help/how-do-i-download-the-software-i-purchased-at-turb...

Or you might have another account.  It's common to end up with multiple accounts. First LOG OUT of whatever TurboTax account you're logged into right now. Then use this TurboTax account recovery website to get a list of user ID's for an email address. Run the tool against any email addresses you may have used
https://myturbotax.intuit.com/account-recovery/