You can only print and mail the 2018 tax return after October 15, 2019 if you did not e-file prior to that date.
See this TurboTax support FAQ for the procedure to print and mail a tax return when using the online editions - https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online...
