How to I add on that I got the 6 months of child tax credits and now I'm applying to get the other half 1800?

You were asked when do your return if you received the advance child tax credit in 2021.  If you answered the question and entered the $1,800 you received in 2021 then the remaining $1,800 Child Tax Credit will be on your federal tax return, Form 1040 on Line 28.  The credit will be included with your tax payments and will be part of your federal tax refund.

Look at your federal tax return, Form 1040 Line 28.  Is there an amount entered?

To access your current or prior year online tax returns sign onto the TurboTax website with the User ID you used to create the account - https://myturbotax.intuit.com/

When you sign onto your online account and land on the Tax Home web page, scroll down and click on Add a state. 

This will take you back to the 2021 online tax return.

Click on Tax Tools on the left side of the online program screen.  Then click on Print Center.  Then click on Print, save or preview this year's return.

Has your tax return been accepted yet?   If it has been accepted, you can't add that information now.   In this case, you will need to wait until the IRS processes your original return.  They will most likely adjust your refund to not include the payments you already received.   If they don't, you can amend later.   The amending software will be available later this month.  

If you have not filed yet or your return was rejected, you can go back in and add the payments from your 6419 letter.   To do this, search for child tax credit (when you have your return open) and use the Jump to child tax credit link to be taken to the section.  Click through the screens and when you come to the question about advance payments, enter the information from your letter.   If you have already e-filed, you won't be able to make any changes, see above.