That's probably for the Recovery rebate credit on 1040 line 30. Do you have an amount on line 30?   

A common adjustment is if you claimed the Recovery rebate credit on 1040 line 30 when you already got the Stimulus payment or got the second one after you filed.  The IRS knows they already sent it to you.  But you tried to claim it again on your tax return.  

If you claimed a missing stimulus payment on your return but the IRS took it off you have to ask the IRS. They think they already sent it to you. Did you get the 2nd round after you filed your return?   Maybe it went to an account you don't remember. Or you got a check or a card. It was probably easy to miss the debit card in the mail and think it was junk mail. They have to put a trace on it.

How to put a trace on a missing 1st or 2nd Stimulus payment.  IRS for Payment Issued but Lost, Stolen, Destroyed or Not Received

See question F3 here, you can expand it to see how to put a trace on it.

https://www.irs.gov/newsroom/2020-recovery-rebate-credit-topic-f-finding-the-first-and-second-econom...