A common reason is you claimed the Recovery Rebate Credit on line 30 or too much Child Tax Credit on line 28 but the IRS had already sent it to you.
$1,400 is the amount of the 3rd Stimulus payment. Did you already get the Stimulus payment they sent out last year starting March 2021? The IRS has records that show you were sent the 3rd stimulus payment so they removed the $1,400 Recovery Rebate Credit.
