If your parent's insurance was NOT Marketplace insurance, you don't need to enter anything from that other than saying you had insurance for those months.
If you had Marketplace insurance AND was still under your parent's insurance during the same month, you do not qualify for the Premium Tax Credit for that month. That means you need to enter $0 in column B for those months. TurboTax will give you an error for that, and will not let you e-file. You will need to file by mail.
