For a previous year, I'm amending to add the American Opportunity Credit. It says that I do qualify, but it doesn't give me the $1000. What's wrong?

That may be correct, actually. While the full amount of the American Opportunity credit of $2,500 may not be allowed if you don't have any taxable income, you can receive a refundable credit of up to $1,000. So, you don't qualify for the normal American Opportunity credit, but you may be qualifying for the refundable portion which is typically $1,000.