Do you have a qualifying dependent? If not, then you cannot claim head of household.
If so, then did you enter your dependents year? TurboTax will offer your Head of Household status, once you enter enough information to show that you are eligible for it. If you did not enter your dependents yet, you have not given the program enough information to give you that status.
If you did enter your dependents, go back and double check your answers in the personal info section for them and for yourself to be sure you answered all questions accurately.