When you add a 1095A and it shows you owe it is because you received too much of the Premium Tax Credit in advance. When you apply with the Marketplace you are using your estimated income for the year, this is how they determine how much your premium tax credit will be. If your income is more than you estimated income is, you will need to pay back the difference between what you received and what you should have received.
If your income is less than estimated, this may give you a refund of what you should have received but didn't.
So basically, the credit you get is all an estimate that is reconciled when you file your tax return for the year.
https://www.irs.gov/affordable-care-act/individuals-and-families/premium-tax-credit-claiming-the-credit-and-reconciling-advance-credit-payments