Nothing has been filed using the TurboTax 2020 online editions.  Tax year 2020 has not yet completed.  A 2020 tax return cannot be e-filed until late January 2021.

The IRS will not be accepting any type of 2020 tax return until late January 2021.

To add a dependent just click on My Info on the left side of the online program screen.

You could not possibly have e-filed your 2020 return since e-filing is not even open yet.  Just go back to My Info and add the dependent.

And.....based on your hasty mistake just now.....

Please be careful about being in too much of a hurry to file your next tax return. Many tax forms are not available at the very beginning of the tax season, so you have to wait for your forms. The software still needs updates at the beginning of the tax season. 

Although the software will allow you to e-file in early January, the IRS does not begin to accept the e-files until almost the end of January.  So your return just sits on the server.  If you realize that you made a mistake or left something out—as many people do—you cannot change it.

Every year we see people who are in such a rush to file that they file incorrectly and incompletely.  If you file too early and leave out a W-2 or a 1099, or forget to enter information, you will end up having to amend your tax return---and that takes months.  Employers have until February1. 2021 to issue W-2's----and ALL of your 2020 W-2's have to be on the same tax return.  Many of the 1099's and 1098's you need do not arrive until late January or sometime in February.  If you are getting Earned Income Credit or Additional Child Tax Credit, the IRS is going to hold your refund until the end of February no matter how early you file.  There seems to be a greater chance of "early" returns being reviewed and those refunds are delayed, or early returns are more often subject to identity verification--which will also delay processing.  Filing too early may just be a recipe for a delayed refund.  It might go more smoothly for you if you wait until at least mid-February to file instead of e-filing during the first minute that you can do it.

https://ttlc.intuit.com/community/tax-topics/help/how-will-the-stimulus-package-impact-me/00/1393859

At this point, we do not know yet what the new stimulus bill will mean in terms of changes to the 2020 Form 1040, software programming, software updates, etc., etc.  Do NOT be in a hurry to file your 2020 tax return until more is known.  It is never a good idea to file early, and it would be especially foolish to file an early 2020 return.