you got a notice or letter. there should be a phone number on it to call if you have questions. neither Turbotax nor this forum has access to your return or the reason for the state notice.
It's a notice requesting payment. I'll give them a call - I guess I'm confused how the correct amount wasn't calculated when entered into TurboTax in the first place. I initially got a small refund, but now they are requesting underpayment + interest.
3 possibilities and that's the reason you must call. One is a mistake by the state. Such errors do happen and if you don't respond timely you may either lose out on the ability to object or need to go to tax court (think professional representation and other expenses). the other is that while you think your return reported everything and Turbotax correctly calculated the tax based on your entries, the state may have found an error such as unreported income. the third is sort of a mixture. you made a timely tax payment but left off certain identifying information which resulted in the tax payment not being credited to your account. the notice should explain the discrepancy. we may be able to provide a better explanation if you tell us what the discrepancy was but you'll still need to call.