IPtables is not available

Hi all, I’d run cloudmin-kvm-debian-install.sh on my personal Debian Stretch server adding the “–bridge-interface” flag, as the primary interface isn’t eth0 in my case. All went fine till the final IPtables stuff, where I get following error message:

Opening port 10000 on host firewall …
IPtables is not available
… failed

Any hint what the problem can be? Within the installation script I see that ports 10000, 10001, 10002, 10003, 10004, 10005 and 843 would be registered. Why so many ports? Can I add all manually? Where can I find the exact definitions for doing so?

Tx & kind regards.