If you are seeing "Ready to mail," it means you chose the File by Mail option instead of e-File or your return is not eligible for e-filing. So you will need to print and mail your return if you choose to file a paper return or if your return is not eligible for e-file. 

Keep in mind, there are also still some IRS and state forms that have not been finalized yet. You won’t be able to e-file right now if your return has unavailable forms. Click on Review to see if your return has forms that are not yet ready and it should show the date they should become available as well. 

This link will give you the instructions on how to print and mail if you are using the TurboTax Online version of the software and need to mail the return How do I print and mail my return in TurboTax Online?

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 When a tax return can’t be e-filed no matter what you do that would be the cause for not being able to e-file your return. And in 

In this case, you need to print and mail.