What are possible reasons the source and destination showing as N/A in this PPP traffic?

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"PPP" stands for "Point-to-Point Protocol", as in "one and only one host, on one end of the link, is talking to another single host on another end of the link".

As such, PPP has no notion of source and destination addresses; if a host sends a PPP packet, it's the only host that would be sending a packet in that direction, so there's no need for a source address to indicate which host is sending the packet, and it's sending it to the only host that can receive packets sent in that direction, so there's no need for a destination address to indicate to which host the packet is being sent.

For PPP in HDLC-like framing, there is an address field that could, in principle, be used to allow multiple hosts to share a link. I don't know whether it ever is used for that purpose; Wireshark doesn't display its value as the destination address, and there's nothing it can display as the source address, as the HDLC-like header doesn't have a source address field.

For PPP-over-{Ethernet,ATM}, the layer on top of which PPP is running supports multiple hosts, so Wireshark could display Ethernet addresses, or display ATM endpoint information if that's available, but your capture just has PPP in HDLC-like framing.