The Turbo Tax software failed to calculate my Illinois state property tax credit based on my Federal return. If I amend the Illinois State Return will I be charged?

[Critter-3]

There is no cost to amend a return UNLESS you have to upgrade the program you are using ... then you only pay the upgrade fee. 

You should not amend until the first return has fully processed and you get the first refund or your tax due payment has cleared.  

How to Amend

https://ttlc.intuit.com/community/amending/help/how-to-amend-change-or-correct-a-return-you-already-...

[Mike9241]

For tax years beginning on or after January 1, 2017, the Illinois Property Tax Credit is not allowed if the taxpayer’s adjusted gross income for the taxable year exceeds $500,000 for returns with a federal filing status of married filing jointly, or $250,000 for all other returns. 

you may have to go back into the Illinois section of the return to make  sure you entered the questions properly

also review PUB 108 for other reasons you may not qualify

https://www2.illinois.gov/rev/research/publications/pubs/Documents/PUB-108.pdf#search=pub%20108 

can't say if you'll be charged