It informed me of this before it even let me put in my W2's so is this something that works itself out as it goes along? If so that is very poorly designed.
You must pay more than half the household expenses, be single or long-term separated, and provide care in your home for a qualifying person who is usually your child dependent. If you have entered your own personal information and your dependent information, and the program will not recommend or allow you to switch to HOH status, then there is a problem with the dependent. The dependent must live in your home more than half the year. If you picked "6 months", the program understands that to be exactly half the year and exactly half is not more than half. Try changing to 7 months or more. (If the child does not live in your home more than half the year, they do not qualify you for HOH even if you have a custody order that allows you to claim them as a dependent.)
You may need to delete the dependent and enter them again and double check your answers.