No one at TurboTax knows what happened with your IL return. The state does not provide any information to TurboTax after you filed your return. Sounds like you received a letter from the state. Read it more carefully. What does it say?
You can get some help to understand it:
Once a tax return has been filed and accepted, TurboTax receives no further information from either the IRS or a State concerning the tax return.
The state notice you received should explain exactly why the state has determined that you owe taxes for tax year 2017. There should be procedures for you to follow if you agree or disagree with the assessment. There should also be a phone number for you to call if you have questions.
You can also use the TurboTax Audit Support Center and speak with a support agent who can assist you with understanding the notice you received.
Support is open from 5am to 5pm Pacific time, Monday thru Friday
Audit Support Center website - https://support.turbotax.intuit.com/irs-notice/audit-support/
One tip: if you choose to protest the additional tax assessment from Illinois, be sure to do so by the deadline specified in the letter. Illinois is usually very strict on deadlines.