Is it possible to still e-file my 2018 tax return? I prepared everything last year but did not file. Can I still e-file or do I need to mail it in?

You need to mail it in, you cannot e-file a prior year return with TurboTax.  Make sure you sign the return.

To access your current or prior year tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/

Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show.  Click on the Year and Click on Download/print return (PDF)

A 2018 tax return can only be printed and mailed, it cannot be e-filed.

Print, sign, date and mail the tax return to the IRS.  Include with the mailed return any forms W-2 and 1099 which have taxes withheld.  Go to this IRS website for mailing addresses - https://www.irs.gov/filing/where-to-file-paper-tax-returns-with-or-without-a-payment

There are no penalties for filing a tax return after the due date if filing for a tax refund or there are no taxes owed.