10b is the "above the line" charitable contribution deduction that you get if (1) you made $300 or more in monetary charitable contributions and (2) you are taking the standard deduction. If you entered charitable contributions either in the Deductions section OR in response to the specific prompt about this $300 deduction, that is where the number came from.
Sometimes you'll get an error if the amount was marked as "estimated." Go to Forms (upper right on the main screen), click on 1040, then right click on the problem number in line 10b. You can see if it is marked as estimated, or you can override the number from here.
If you made no charitable contributions, OR you are itemizing, then you can just change that number to 0 (if you are itemizing, then all of your charitable deductions should be on Schedule A). If you made charitable contributions, but in an amount less than $300, override the number on 10b to reflect the true number.
