It is NOT on your 2020 return. If you don't have the letter 6419 you need to check your bank account to see how much Advance they sent you in 2021 (July-Dec). And please stick with your first question. You should not be starting new posts. Add on to the existing one.
The new Child Tax Credit for 2021 raises the cut off age from 17 to 18 and makes the credit $3,000 (3,600 for a child under age 6). Check 1040 line 28 for it. You might have got the Advance Child Tax Credit for Half during the year. You need to enter what you already got so you don't try to claim it again.
Line 30 and 28 is only if you didn't get the full amount or qualify for more.
You should also get letters 6475 and 6419 showing what the IRS sent you. Turbo Tax blog on IRS Letters 6419 and 6475
IRS Letters 6419 and 6475 for the Advance Child Tax Credit and Third Stimulus: What You Need to Know...
