Prior to filing, Stash said I don't have tax docs for 2018.They just sent me a 1099. How do I update my taxes for 2018 so it doesn't appear that I've committed tax fraud?

If you have just filed the 2018 taxes and now have a 1099, the return will need to be Amended and the 1099 added.

Once the return has been filed it cannot be stopped (unless you get extremely lucky and IRS would reject it for some reason).

So you have to wait for the return to be processed and you get your refund.

Then approx February 22nd you can sign back into TurboTax and do the Amended return.

Thank you