2020 amended return rejected. Called IRS support, the 105C letter was not received. Verified address, 2nd 105C not received. Can I get an advocate to help?

Amended returned received in October 2023. It was rejected in December 2023. Contacted IRS 2/9/2024, verified address and requested 105C to be resent. It has not been received as of this date, 3/23/2024. Can I get an advocate to help resolve this?