I have completed the turbo tax form for mailing but have not done so. I have received a 1099 to include. How can I do that?

If you did not file the return electronically, simply tear up the printed copy, go back into the income section of TurboTax and enter the missing 1099 and reprint for filing. If you filed electronically you will have to amend the return on 1040X. Do not do that until your original return has been processed by IRS i.e. you receive any refund due. 

Click on Add A State to let you back into your return.

Since you haven't filed you return you can go back in and add the 1099.  To get back into your return after printing or e-filing, please follow these steps:

1. In TurboTax Online, sign into your account.
2. Select Tax Home in the left menu.
3. Scroll down to Your tax returns & documents.
4. Select 2019, and then select Add a State (you're not actually adding a state, this just gets you into the right menu).
5. Once you're in your return, you can navigate around as usual.