You can file your 2018 tax return now, but you can't do it with TurboTax Online. TurboTax Online is only for the current tax year, which is now 2019. To file for 2018 you have to use the CD or download TurboTax software for 2018. You can purchase a download at the following link. There is no free filing for past years.
Past Years' Products
You might get a better price from a reputable online seller that still has TurboTax for 2018 in stock.
Tax returns for past years cannot be e-filed. You will have to print your 2018 tax return, sign it, and file it by mail. At the present time, because of Covid-19 restrictions, the IRS is not processing tax returns that are filed by mail. When they start processing paper returns again they will have a big backlog, so it will be quite a while before your 2018 tax return is processed.