If I didn't receive as much advance child tax credit as the IRS is reporting, what amount should I report?

If Letter 6419 that you received from the IRS for the advanced child tax credit is different than the amount you received, the first thing to do is check the IRS Child Tax Credit Update Portal at Child tax Credit Update Portal, which should show a record of all payments sent to you.    

If the portal shows that the IRS sent a payment that you didn’t receive, you will have to go through a few extra steps to get the correct information. You should request that the IRS trace the missing payment, which can help the agency determine if it was delivered to you or returned. If the payment wasn’t delivered, the IRS record should be updated, and you can exclude the missing check in your total when you file and claim it to get it back.

You could make the adjustment, in the amount you report on your tax return, but it may delay your refund.  The IRS may also deduct from the refund any amounts they show you received but didn't receive.   The discrepancy is something you will need to work out with the IRS.