When you install isc-dhcp-server, you need to copy the config files as isc-dhcp-server6 in each area. 


# DHCPDv4_CONF=/etc/dhcp/dhcpd.conf
# DHCPDv6_CONF=/etc/dhcp/dhcpd6.conf

# Path to dhcpd’s PID file (default: /var/run/dhcpd.pid).
# DHCPDv4_PID=/var/run/dhcpd.pid

# Additional options to start dhcpd with.
# Don’t use options -cf or -pf here; use DHCPD_CONF/ DHCPD_PID instead

I added a DHCP server to my primary DNS server.  I enabled IPv4 and IPv6.

Setting up host addresses for IPv6 is tricky.

host ubuntu Server_Name {
host-identifier option dhcp6.client-id Long_ID_String;
fixed-address6 IPv6 address;

How to get

host-identifier option dhcp6.client-id?
I opened the DHCPv6 server. 

sudo systemctl -l isc-dhcp-server6

Go the the server that you want to find the information on

sudo netplan try, enter, sudo netplan apply

Load sudo systemctl -l isc-dhcp-server6 again.  You will see it on the screen.  Copy the string into /etc/dhcp/dhcp6.conf for that server.

Sudo systemctl restart isc-dhcp-server6 or sudo reboot.  do try and apply again and ifconfig to check