Online is closed now to start a new 2018 return. Oct 15, 2019 was the last day to efile. You need to buy the 2018 Desktop Download to do 2018 now. You can buy 2018 Desktop download on the prior year products page here,
https://turbotax.intuit.com/personal-taxes/past-years-products/
You now have to print and mail 2018. Be sure to attach copies of your W2s and any 1099s that have withholding on them. You have to mail federal and state in separate envelopes because they go to different places. Get a tracking number from the post office when you mail them for proof of filing.
How to print and mail from the Desktop program
https://ttlc.intuit.com/community/print-and-file/help/how-do-i-print-and-mail-a-return-in-the-turbotax-for-windows-cd-download-software/00/26256