The TurboTax online editions are Only for tax year 2019.
To complete and file a 2018 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2018 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2018 tax return can only be printed and mailed, it cannot be e-filed.
Yes you can-- however you can not file electronically. You will have to finish your returns (2018), and then file them by regular mail. You can get the proper address to send them physically to from irs.gov under where to file my return. I would suggest that you include (at the back of the return) a separate statement for the reason you are filing late. If you did not owe any taxes last year this should not be a problem-- if you did you may incur interest and penalties; if this is the case go ahead and file your return, the Internal Revenue Service will advise you (in writing) of any taxes and penalties.