At some point NAT66 prefix translation has stopped working on the 1.5 nightly.
Here's a sample config and ip6tables output showing nothing configured based on the config.
fahad@vyos-vps:~$ show config commands | grep nat66
set nat66 destination rule 9001 description 'Wireguard Incoming NAT66 prefix translation'
set nat66 destination rule 9001 destination address '2a02:c206:2076:2040::a:0/112'
set nat66 destination rule 9001 inbound-interface name 'eth0'
set nat66 destination rule 9001 translation address 'fd99:d781:643e:8308::a:0/112'
set nat66 source rule 9000 description 'Wireguard Outgoing NAT66 prefix translation'
set nat66 source rule 9000 outbound-interface name 'eth0'
set nat66 source rule 9000 source prefix 'fd99:d781:643e:8308::/64'
set nat66 source rule 9000 translation address '2a02:c206:2076:2040::/64'
fahad@vyos-vps:~$ ip6tablew
Invalid command: [ip6tablew]
fahad@vyos-vps:~$ ip6tables -nvL
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
fahad@vyos-vps:~$ ip6tables -t nat -nvL
Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination