Ready to mail means that you either selected to print and mail your tax return or that you are unable to efile and must mail your tax return to the IRS. See the link below for more information.
https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online/00/26258
As long as you haven't already mailed your 2019 paper return, you can switch to the e-filing method. See the instructions in the following link to do so.
https://ttlc.intuit.com/community/e-file/help/how-do-i-change-from-mail-to-e-file-in-turbotax-online/00/26985