Your version of Squid is not supported by Webmin. Only versions from 1 to 6 are supported by this module.
The command squid -v returned :
Squid Cache: Version 7.1
Service Name: squid
This binary uses OpenSSL 3.0.13 30 Jan 2024. configure options: ‘–prefix=/usr’ ‘–bindir=/usr/bin’ ‘–sbindir=/usr/sbin’ ‘–sysconfdir=/etc/squid’ ‘–with-default-user=proxy’ '–wit