This error means that you are using an operating system that is end of life so ClamAV is no longer providing signature updates for that OS. These OS include CentOS 7.x/CloudLinux 7.x/RedHat Enterprise Linux 7.x/Debian 10.x/Ubuntu 20.04 running ClamAV 0.103.0. ClamAV stopped providing anti-virus signature downloads for operating systems on Sept 14, 2025. See here for more information.
Upgrade Your Operating System
To fix this issue you should upgrade/migrate your server to a supported OS. Centos 7 OS can perform an in place upgrade to AlmaLinux 8, Ubuntu/Debian OS can perform a dist-upgrade to a new OS version, all other OS can use the Plesk migrator to migrate sites over to a server running a supported OS.
Centos 7 users doing an in place upgrade will also have to perform additional tasks after upgrading. See here for more information.