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
See the instructions in the following link to change from mail to e-file.
https://ttlc.intuit.com/community/e-file/help/how-do-i-change-from-mail-to-e-file-in-turbotax-online/00/26985
Some tax returns are not able to be e-filed. See the link below for assistance.
https://ttlc.intuit.com/community/e-file/help/when-a-tax-return-can-t-be-e-filed-no-matter-what-you-do/00/26142