Enable IPv6 on existing Ubuntu Droplet

I followed this tutorial to enable IPv6 on my existing Ubuntu 18.04.3 LTS Droplet. My server doesn’t have anything in /etc/netplan, so I followed instructions for Ubuntu 16.04 and change /etc/network/interfaces. I add suggested stuff (changed with IPv6 addresses I got on DigitalOcean dashboard while enabling IPv6). After the service restart, I got:

Dec 16 06:00:41 localhost systemd[1]: Stopping Raise network interfaces...
Dec 16 06:00:41 localhost dhcpcd[1073]: eth0: deleted default route via 46.101.128.1
Dec 16 06:00:41 localhost dhcpcd[1073]: eth0: deleted IP address 46.101.132.77/18
Dec 16 06:00:41 localhost dhcpcd[1073]: eth0: deleting route to 46.101.128.0/18
Dec 16 06:00:41 localhost ifdown[1457]: RTNETLINK answers: No such process
Dec 16 06:00:41 localhost ifdown[1457]: RTNETLINK answers: Cannot assign requested address
Dec 16 06:00:41 localhost dhcpcd[1073]: eth0: arp if_readrawpacket: Network is down
Dec 16 06:00:41 localhost dhcpcd[1073]: eth0: carrier lost
Dec 16 06:00:41 localhost systemd[1]: Stopped Raise network interfaces.
Dec 16 06:00:41 localhost systemd[1]: Starting Raise network interfaces...
Dec 16 06:00:41 localhost dhcpcd[1073]: eth0: carrier acquired
Dec 16 06:00:41 localhost dhcpcd[1073]: eth0: IAID 86:e5:62:01
Dec 16 06:00:41 localhost dhcpcd[1073]: eth0: adding address fe80::88e6:3b61:12cd:fe3d
Dec 16 06:00:41 localhost dhcpcd[1073]: eth0: using static address 46.101.132.77/18
Dec 16 06:00:41 localhost dhcpcd[1073]: eth0: adding route to 46.101.128.0/18
Dec 16 06:00:41 localhost dhcpcd[1073]: eth0: adding default route via 46.101.128.1
Dec 16 06:00:41 localhost ifup[1544]: RTNETLINK answers: File exists
Dec 16 06:00:41 localhost ifup[1544]: Failed to bring up eth0.
Dec 16 06:00:41 localhost systemd[1]: networking.service: Main process exited, code=exited, status=1/FAILURE
Dec 16 06:00:41 localhost systemd[1]: networking.service: Failed with result 'exit-code'.
Dec 16 06:00:41 localhost systemd[1]: Failed to start Raise network interfaces.
Dec 16 06:00:42 localhost dhcpcd[1073]: eth0: soliciting an IPv6 router
Dec 16 06:00:55 localhost dhcpcd[1073]: eth0: no IPv6 Routers available

1. Any idea what can be wrong?
2. Why my server still uses the old way of networking (and not netplan)? I didn’t remember, maybe it was upgraded?

Regards.