If you chose the File by Mail option by mistake, you can switch to e-file using the steps below. Click on "Change to e-file your return in TurboTax Online versions" below and it will show you the instructions for how to do that. 

NOTE: If you try to switch to e-file and it doesn't allow you to, you can find some reasons listed here https://ttlc.intuit.com/questions/1901497 that would be the cause for not being able to e-file your return. And in this case, you need to print and mail. This link will give you the instructions on how to print and mail if you are using the TurboTax Online version of the software https://ttlc.intuit.com/questions/1944348