**For IL 1040-8100-1 Error**
This error happened because you're using your previous year's Federal Adjusted Gross Income as a signature. Illinois checks the number your provided against a database. If it matches it's accepted; if it doesn't it's rejected.
Two options to fix this. For either one, you want to go to File in Turbotax, as in the place where you submit your taxes to file electronically. Keep clicking through. You'll reach a page where it asks for a signature.
By default, Turbotax wants you use your previous year's AGI, and it has auto-populated this value. If you're getting IL1040-8100-1 error, this value is probably wrong.
Option 1: Fix the value. It might be wrong for any number of reasons; I don't know your life. In my case I was able to find the corrected value in a notice Illinois sent me. I found this by logging in to https://mytax.illinois.gov/ I went to Action Center -> Letters and found a letter they'd sent me with an updated AGI. I input that number instead; it worked.
So that was hard. Here's an easier way.
Option 2: Uncheck the box in Turbotax that uses AGI. You'll need your IL-Pin, which you can get by going to https://tax.illinois.gov/individuals.html and clicking on "Get your IL Pin." You'll need your social security number and driver's license number.
