we can't see the return but the taxpayer should have gotten a letter of explanation with the initial and subsequent bill. if you have a POA you can call the Illinois Department of Revenue to get an explanation otherwise only the taxpayer can call. If you are a paid preparer you have violated Turbotax's terms of service. yours appears to be the only thread about this "issue". therefore it is highly probable there was an error or omission in preparing the return, not a computational error.
the PTC requires the County and PIN number perhaps one or both are incorrect. did you submit Schedule ICR?
in addition, only Illinois real estate property taxes can be used and only if not taken as a business/rental deduction on the federal return.
