It looks like the postinst script is failing because the test -e on line
15 looks to a broken symlink at /sbin/iptables (at least on my system).
Changing the -e to -L to test for a symbolic link could work.
Changing the /var/lib/dpkg/info/iptables.postinst file on line 16 to add
the -f option to ln (to force overwriting) also could work:
ln -sf /usr$i $i
After running dpkg --configure -a it said:
Setting up iptables (1.8.2-1) ...
update-alternatives: warning: forcing reinstallation of alternative
/usr/sbin/iptables-nft because link group iptables is broken
update-alternatives: warning: forcing reinstallation of alternative
/usr/sbin/ip6tables-nft because link group ip6tables is broken
update-alternatives: warning: forcing reinstallation of alternative
/usr/sbin/arptables-nft because link group arptables is broken
update-alternatives: warning: forcing reinstallation of alternative
/usr/sbin/ebtables-nft because link group ebtables is broken
