States have different rules and procedures when it comes to extensions – some require them, some don't, and some don't even have an extension form.
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.
Virginia (VA) – The filing deadline for Virginia personal returns or extensions is May 1. Virginia automatically gives you a 6-month extension to November 1 if you already paid or withheld at least 90% of your total Virginia tax liability. If you still owe more than 10% of your total tax liability, mail Form 760IP or e-file your extension by May 1. Include a check with your Form 760IP or pay when you e-file your extension to avoid penalties.