I was able to solve it after so many tests, for some reason the kernel.modules_disabled option was disabled, making it impossible to load the iptables modules.
Greetings, I have installed on a vps with ubuntu 18.04 and plesk obsidian 18.0.36, I am creating custom rules and it tells me that they cannot be activated.
trying to activate new firewall rules throws the following errors.
# /usr/local/psa/var/modules/firewall/firewall-active.sh
iptables...