In the online version its under Self-employment income and expenses -> [Your Business] -> Expenses -> Less Common -> Health insurance premiums. Be sure to go through all pages of the list. Then edit the Health insurance premiums item under Expenses.
In the desktop version it's under Business Income and expenses (Sch C) -> All other expenses -> Self-Employed Health Insurance Premiums.
@bwoirhaye , "other common business expenses" / "insurance payments" is only for other types of insurance payments that are deductible on Schedule C. It is not where you are to enter self-employed health insurance payments.
