Check your online account again and make sure it says your 2017 was accepted.
Note: If it says “Ready to Mail” or “Printed” that means YOU had to mail it yourself. TurboTax does not mail your tax return for you.
Then read the letter from the IRS again. Does it say you did not FILE your return or that you did not PAY your tax due? Those are not the same thing. If they say you did not pay tax owed, your only proof is your own bank or credit card record.