Deductions & credits

You must have enough income to have a tax liability that can be reduced by the credit.  Unless there is a tax liability there is nothing that the credit can reduce.

The child must have been under age 17 at the end of 2015.

The amount of child tax credit you can receive is limited by your tax liability.

If your tax liability is zero, then you are not eligible for any child tax credit.

However you might be eligible for the Additional Child Tax credit.

For example: 

If you are single with one child, your standard deduction and two exemptions will be  $14,300 ($6,300 standard deduction plus $8,000 exemption for you and your child), which means that you must have more earned income than $14,300 before you have any tax liability in order to get the Child tax credit.  ($24,600 if filing jointly with one child)

However, if you qualify for the Child Tax Credit but do not receive the full amount, and if your earned income is greater than $3,000, you might be eligible for the Additional Child Tax credit.  The amount will vary from $1 if your earned income was $3,004, up to $1,000 if your earned income was $9,666 or more.

Too high of an income will reduce or eliminate the CTC also.  For a single taxpayer (or HOH) the phase out starts with a AGI of $75,000 (MFJ $110,000, MFS $55,000).

The actual amounts on your tax return will depend you your filing status and the number of child dependents.