```
$ apt-get source ifupdown
[...]
$ grep -rB2 'No DHCPv6 client software found' ifupdown-0.8.44
ifupdown-0.8.44/inet6.defn-    dhclient -6 -1 -v -S -pf /run/dhclient6.%iface%.pid -lf /var/lib/dhcp/dhclient6.%iface%.leases -I -df /var/lib/dhcp/dhclient.%iface%.leases %iface% \
ifupdown-0.8.44/inet6.defn-        elsif (var_true("dhcp", ifd) && execable("dhclient"))
ifupdown-0.8.44/inet6.defn:    echo 'No DHCPv6 client software found!' >&2; false \
[...]
```

It's looking for `dhclient`, not `dhcpcd`.

In `inet.defn`:
```
method dhcp
  description
    This method may be used to obtain an address via DHCP with any of
    the tools: dhclient, udhcpc, dhcpcd.
```

But for `inet6`, that's `dhclient` only.

It's found in:

```
$ apt-file search -F /usr/sbin/dhclient
isc-dhcp-client: /usr/sbin/dhclient
isc-dhcp-client-ddns: /usr/sbin/dhclient
```