Did you receive the 1st EIP payment in the same account as you receive your SS benefit payments? If so, the IRS would have sent the 2nd EIP to the same account and you would have received an IRS Notice 1444-B for the second payment.
The IRS probably denied your Recovery Rebate Credit on the 2020 tax return because they have records showing that you received the payment.
If the payment was never received by you a trace can be put on the payment by the IRS.
See this IRS website - https://www.irs.gov/coronavirus/second-eip-faqs#paymentlost
