If you didn't file a tax return last year, you will be able to indicate that on the AGI screen. Don't enter your 2017 AGI or your Efile will be rejected.
Click the link for more Info on Entering your AGI.
**Mark the post that answers your question by clicking on "Mark as Best Answer"
When you tell the TurboTax program that you didn't file a 2018 return, it should enter a 2018 AGI of 0 (zero) for you in the background. If it doesn't, or if you entered some other figure by mistake, you can manually enter a 0 in the AGI field.