Hi,
Kernel Debian 4.19
When I perform
iptables -A syn_flood
and
"-A" appends a rule to the specified chain. In your case chain names are specified "syn_flood" and "port-scanning". Error "No chain/target/match by that name" means there are no such chains, and they should be explicitly created before adding a rule to each of them. A new chain is created by "iptables -N chan_name".
This question is more related to iptables and its syntax, than to kernel.