You shouldn't need to enter zero for income. Just don't enter any income. Income goes on Schedule C line 1. I. It probably doesn't fill in zero, just leaves it blank.
You can show a loss on schedule C.
Some expenses, such as home office or section 179 depreciation can only be used to reduce your schedule C taxable income to zero, and not to create a loss. Excess deductions for these carry over to the next year. And you have to answer yes to both questions about exclusive and regular use, not just one. The area of your home office must be used regularly and exclusively for business to deduct it.
Or you checked the box on 32b saying Some Investment is Not at Risk.
