If my transmission didn't go through because a tax return w/ the same social security number has already been submitted, but I haven't submitted before, what should I do?

If it thinks another return has been been filed then you have to print and mail your return and let the IRS sort it out.

 If a ssn on your return has been used on another return it could be just a mistake and someone else accidentally typed the wrong ssn on their return.  Or if your dependent filed a return did he check the box that says he can be claimed on someone else's return?

How to switch from efile to mail

How do I switch from "E-File" to "File by Mail" in TurboTax Online?

How to mail a return from the Online version

https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online/00/26258

Be sure to attach copies of your W2s and any 1099s that have withholding on them.  You have to mail federal and state in separate envelopes because they go to different places.  Get a tracking number from the post office when you mail them for proof of filing.