Yes, you can file your 2018 tax return now. However, it cannot be e-filed as only 2019 tax returns can be e-filed this year. You will need to buy a separate program for 2018.
File your 2018 taxes
If you are getting a refund, there is not a penalty for filing past the deadline. You have 3 years to file for a refund. If you owe taxes, the interest/penalties will be calculated by the IRS based on how much you owe and when they receive your return and payment. The IRS will bill you for this; it will not be calculated by TurboTax.
How do I prepare a prior-year tax return