some notes on this on how to use e.g. netstat to see if the slave is actually listening on the expected port are provided in the blogpost:
https://blog.haardiek.org/setup-openvas-as-master-and-slave.html
i had linked in Setting up Remote Scanners with Centralized Manager - #4 by cfi - Greenbone Community Edition - Greenbone Community Forum
Tools like nmap and similar could help you as well to see if the master is able to reach the expected port on the slave.