The dependent care credit is disallowed for married filing separately. If the program does not think you have a qualifying child dependent to file as HOH, you must have entered something incorrectly, or else the child really doesn't qualify this year.
Your child qualifies you as a dependent for HOH purposes if they are under age 19, or full time student under age 24; AND they don't earn more than half their own support; and they live in your home more than half the year; AND you pay more than half the total living expenses of the home where you live with your qualifying person. They also have to be US citizens or residents. As I stated below, check their birthdate, and how long you said they lived with you. Also check the support question, it should not ask who else paid their expenses, it should only ask if they paid more than half their own expenses (this is unusual, think child actors, and similar situations.)
And check your own interview, did you indicate that someone else paid more than half your living expenses?
We can't see your file so we can't see which answer is disqualifying you or your child.
