Line 30 on the 2021 Form 1040 is used to claim the Recovery Rebate Credit. If you have received your full payment, leave it blank. If you received none or a partial payment, enter the amount you received.
To receive the Treasury notice, update your address by filing Form 8822 and call 1-800-829-1040 to request a duplicate notice after filing Form 8822. You can also check your IRS account online and view transcripts and payments, which might provide additional information. Access your IRS account here.