It appears you have indicated that an adjustment needs to be made to the amount of Capital Gains reported on your federal return. Line #15 reports - Adjustment from Wisconsin Schedule T.
Go back through your WI state return (not just the review) and review your entries. Click on State in the left side menu and select the option to go back through your WI return. When you get to the screen that says ''Here's the income that Wisconsin handles differently'', scroll down and click start or revisit next to Investments, Wisconsin Capital Gains and Losses, see image below. Then carefully review your answers. This is the section that is generating your error.
From the instructions for Schedule WD - If you have a difference in the Wisconsin and federal basis of property (other than your principal residence) and that property is
a capital asset (sale or other disposition is reported on federal Schedule D), fill in the federal gain or loss in Part I or Part II of
Schedule WD, as appropriate. You must also complete Part I of Wisconsin Schedule T to compute the amount to fill in on line 6 or
15 of Schedule WD.
