Hi!
As you are learning it would be better to follow all the instructions as in the lectures, I mean using custom kali and the syntax as showed by Zaid, I reapeat this is for learning purposes, and once the scripts work you can use your custom Debian machine and the syntax you want, this will also help to spot the error in your custom script. I don’t know the specifics of your custom Debian so I can’t tell if there’s any networking issue, did the DNS and file interceptor worked as expected?
Can you share the result of iptables -L and iptables -t nat -L after setting the rules for INPUT and OUTPUT?
Thanks!
Diego