A state extension needs to be filed separately.
If you already completed (or are mostly finished with) your 2018 state return in TurboTax Online, go back into your state and look for a Need more time to file? or similar extension question or option, usually towards the end of the interview (step through your state again from the beginning if you didn't find it). Then follow the onscreen instructions in TurboTax.
Otherwise, if you haven't (or you just) started your 2018 state return, scroll down to your state for alternative instructions.
https://ttlc.intuit.com/replies/3300227For Federal, if you don't have your AGI, or your extension gets rejected for the wrong AGI, here are 2 options:
If you filed prior to the deadline, but were rejected, you have until April 20 (April 22 for Maine and Massachusetts residents) to mail in your extension.