When did you get the 1099G? If you got it in 2018 why didn't you enter it on your 2017 return?
But you might not need to enter it. Or if you did it might not be taxable.
A State Tax Refund is taxable if you itemized deductions on that prior year's 2016 federal return and took a deduction for state income taxes instead of the sale tax. You got a deduction benefit for it so now you have to include it as income. If you took the standard deduction or filed a 1040EZ or 1040A for 2016 it is not taxable and you don't need to report it.
