The error message is not referring to line 11 of Form 1040. It's line 11 of the Deferral Smart Worksheet for Schedule 3 line 12e. To get rid of the error, don't enter anything on line 11. When the error comes up in Smart Check, delete the zero (or whatever is there) and leave the box empty.
If line 10 of the Smart Worksheet is negative, you are not eligible to defer any of the Social Security portion of your self-employment tax.
Zero is too large because it cannot be more than the amount on line 10. Line 10 is negative, so zero is higher. But the solution, as I said, is to not enter anything, and leave line 11 blank.
