You will not get arrested for forgetting to file your 2018 tax return.
You can still file for 2018, but not 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.