Schedule 8812 question

I have been filing as Single for years, and don't even know what is Schedule 8812. For my 2021 filing, during the final review, TurboTax asks me to:

"Check This Entry

Schedule 8812: Advanced payments received must be entered

Advanced payments received __________"

 

On the Your Personal Info Summary page, Marital Status is Single, Dependents is No dependents, and Filing Status is Single. 

 

Why is Schedule 8812 necessary? I have not received a Letter 6419. Please help me making the B1entry.

 

Thanks in advance.

schedule 8812.jpg