The IRS doesn't send letters about state tax due. Was it from your state? You return on Turbo Tax doesn't have an actual signature. You signed it by enter the previous year AGI to verify you. If you need a signed copy you have to print it out and sign it in ink yourself.
If you efiled there should be an Electronic Postmark page saying when you filed and when the IRS accepted it. Probably near the end. That is your proof and signature.
They're asking a 2018 state tax due? That's pretty old. They would have sent letters before now. Are you sure it's from the state? Could be a scam.
Your only proof of payment is your bank statement or credit card showing the payment coming out. Turbo Tax only passes your bank account number to the state for the state to take it out of your account. But most states do not let you pay by Direct Debit so you have to mail in a check or go directly to the state's website to pay. And better double check your bank account and routing number with your bank. Some banks use a different routing number for deposits and payments. And you can't have the state tax due deducted from your federal refund. Federal and state are completely separate.
Check the printout or PDF of your return; look for the state cover sheet with the Turbo Tax logo. If you owed tax, it will show the payment information and how/when you decided to pay. Read the state payment instructions carefully since most states cannot be paid from within the TT system and requires additional steps.
