The CTC is a non refundable credit. You can only claim it if you have a tax liability, which you would not since your income is less than your standard deduction.
The ACTC is $42 because that is calculated based on income. To calculate it, you would subtract $2,500 from your earned income and multiple that by 15%. So in your case it would be 15% of $278 which is $42 would be your maximum credit.
What is the Additional Child Tax Credit?