is your Illinois adjusted gross income more than $250,000 if not married or more than $500,000 if married filing jointly?
are you a nonresident?
then you get no deduction.
only your principal residence in Illinois qualifies.
credit cannot exceed your Illinois tax liability (it's nonrefundable)
if less than those amounts, have you answered yes to all 4 questions concerning your principal residence.
have you entered the county and index number?
if none of these apply contact Turbotax support