Make sure your Dependents are entered correctly in the My Info section. Children 17 and over don't qualify, or if they did not live with you over half the year.
You need to indicate that they 'did not provide over half their support' in that section, too.
The Child Tax Credit you receive is first applied to any tax you owe.
Here's details about how the credit is figured to help you:
https://ttlc.intuit.com/replies/3301702
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