Can you clarify your question? The IRS is adding any missing stimulus payments as a Recovery Rebate Credit to the 2020 income tax return. If you file your relative’s Form 1040, the missing stimulus payment will show up on line 30.
If you mean you went to Get My Payment and it showed that the IRS sent out a payment and it was never received, then you can request a trace by calling the IRS at 800-919-9835 or mailing or faxing a completed Form 3911, Taxpayer Statement Regarding Refund PDF.
What is the form I need to obtain a missing stimulus check?
Any missing stimulus payments from rounds one and/or two can be claimed as a Recovery Rebate Credit on a 2020 tax return. TurboTax will complete all the forms you need based on your answers and inputs to the interview questions in the program. There is no special form to file for the Recovery Rebate Credit other than the basic Form 1040.
Here is a link with more information on the Recovery Rebate Credit.
If you want to start a return you can begin here for free.