The IRS letter 6419 is the official documentation that has the details to be reported for a taxpayer’s advance Child Tax Credit (CTC) payments. This letter will provide:

• Total amount of advance CTC payments received for 2021; this information goes on Schedule 8812
• Number of qualifying children counted in determining the advance CTC

Failure to use the amounts referenced in the IRS Letter 6419 on a customer’s return could trigger a manual review thus possibly delaying the refund by 6-8 weeks.
Please, keep in mind that anyone that received at least one Child Tax Credit payment from July to December 2021 should receive the letter 6419, even if they opted-out of the payments at some point.
If taxpayers cannot provide the IRS letter 6419 but did receive at least one advance CTC payment, there is an IRS Child Tax Credit portal available here to find this information.