iptables -p all --dport

--dport is not a flag for general iptables rules. It's a flag for one of it's extended packet matching modules. These are loaded when you use -p protocol or -m. Unless you specify -m <protocol> or -p <protocol> with a specific protocol you can't use --dport

You'll see this within the iptables(8) or iptables-extensions(8) manual page:

   tcp
       These extensions can be used if `--protocol tcp' is specified. It provides the
       following options:
       ...
       [!] --destination-port,--dport port[:port]
              Destination port or port range specification.  The flag --dport is a
              convenient alias for this option.
       ...

Not all protocols have a --dport flag because not all protocols support the notion of ports

'all' encompasses more than just TCP and UDP; it also covers protocols like ICMP which have no concept of port numbers, and thus can't take a --dport parameter.