TurboTax IL-State has a bug (deluxe version) which results in you getting $0 credit for tax paid to other state. It does not complete IL-1040 Schedule-CR correctly, it left column B empty, resulted in ratio of 0, tax credit 0.
We had w-2 income and paid tax to CT in 2015. In TurboTax IL state interview "Credit for taxes paid to another state" section, I entered the income and the tax paid to CT. TT showed credit is 0. The IL-1040 line 16 was 0.
I thought this was some how questionable. So I did some digging on http://tax.illinois.gov/, then realized that TurboTax generated return did not even have Schedule CR which is where the credit should be figured out.
I got into TurboTax, checked that I have answered the interview questions correctly.
When I took a look at the forms directly, the Schedule CR had the data I have entered in a worksheet, it had the numbers from Fed1040 in column A, but the column B (non-Illinois portion) is all empty.
Once I manually entered the income from CT to line 1 column B, the table automatically calculated the ratio which is now non-zero.
As long as your tax to IL and to other state are both non-zero, with non-zero ratio, you will get some credit. In my case, it was $352 credit. Now I have to file amend IL-1040-X.
Hope this will help some one. Sorry I only found out this after the tax due date.
TurboTax is the tax software I have used for many years. I generally trusted it. This mistake is a huge disappointment especially when I saw people who had similar questions posted there many months ago.
The problem seemed still not resolved. I know last a few years TT had issues with customer support. This sort of mistakes make me thinking of moving away from it completely.
