/etc/ssh/sshd_config.d/40-redhat-crypto-policies.conf includes /etc/crypto-policies/back-ends/opensshserver.config, which specifies
KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512
The mlkem768x25519-sha256 algorithm is not included in this list.
Based on https://www.redhat.com/en/blog/post-quantum-cryptography-red-hat-enterprise-linux-10, you need to run the following commands:
dnf install crypto-policies-pq-preview crypto-policies-scripts
update-crypto-policies --set DEFAULT:TEST-PQNote that the package and system-wide cryptographic policy name are subject to change when post-quantum cryptography exits the technology preview state.