tcpdump: WARNING: arptype 65535 not supported by libpcap - falling back to cooked socket
cannot find user pcap
What does this mean ^^ and how can it be fixed?
The error message is saying that libpcap can't access the raw socket (so would not be able to tell it to switch to promiscuous mode). Usually a permissions issue (ie running as a non-root user).
The reference to the pcap user suggests to me that your version of libpcap supports the use of a specific user with rights to the raw sockets (presumably called 'pcap').