your total net profit (self- empolyment) in Schdule C (or CE) Could not be nagative.
For example, in this year your total cosulating income is 50, your expenses (travel, etc) are 70. Then your net profit is zero (put zeron to line 12 in 1040)

Howeverm for rental or rolalitis, you can calim about some amount of loss in 1040 line 17