I enabled private networking for my droplet and configured everything via the guide:

https://www.digitalocean.com/docs/networking/private-networking/how-to/enable/

Unfortunately, I cannot ping the private IP from another droplet and I get an error when I try to restart networking:

> service networking restart
Job for networking.service failed because the control process exited with error code. See "systemctl status networking.service" and "journalctl -xe" for details.

The error:

> systemctl status networking.service

● networking.service - Raise network interfaces
   Loaded: loaded (/lib/systemd/system/networking.service; enabled; vendor preset: enabled)
  Drop-In: /run/systemd/generator/networking.service.d
           └─50-insserv.conf-$network.conf
   Active: failed (Result: exit-code) since Fri 2019-01-11 08:10:49 CST; 21s ago
     Docs: man:interfaces(5)
  Process: 2174 ExecStart=/sbin/ifup -a --read-environment (code=exited, status=1/FAILURE)
  Process: 2168 ExecStartPre=/bin/sh -c [ "$CONFIGURE_INTERFACES" != "no" ] && [ -n "$(ifquery --read-environment --list --exclude=lo)" ] && udevadm settle (code=exited, status=0/SUC
 Main PID: 2174 (code=exited, status=1/FAILURE)

Jan 11 08:10:49 mywebsite ifup[2174]: Failed to bring up lo.
Jan 11 08:10:49 mywebsite ifup[2174]: RTNETLINK answers: File exists
Jan 11 08:10:49 mywebsite ifup[2174]: Failed to bring up eth1.
Jan 11 08:10:49 mywebsite ifup[2174]: /etc/network/if-up.d/sendmail: 44: .: Can't open /usr/share/sendmail/dynamic
Jan 11 08:10:49 mywebsite ifup[2174]: run-parts: /etc/network/if-up.d/sendmail exited with return code 2
Jan 11 08:10:49 mywebsite ifup[2174]: /sbin/ifup: post-up script failed.
Jan 11 08:10:49 mywebsite systemd[1]: networking.service: Main process exited, code=exited, status=1/FAILURE
Jan 11 08:10:49 mywebsite systemd[1]: Failed to start Raise network interfaces.
Jan 11 08:10:49 mywebsite systemd[1]: networking.service: Unit entered failed state.
Jan 11 08:10:49 mywebsite systemd[1]: networking.service: Failed with result 'exit-code'.

My interfaces file is:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

source /etc/network/interfaces.d/*

and /etc/network/interfaces.d/50-cloud-init.cfg:

# This file is generated from information provided by
# the datasource.  Changes to it will not persist across an instance.
# To disable cloud-init's network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
auto lo
iface lo inet loopback
    dns-nameservers 67.207.67.3 67.207.67.2

#auto eth0
iface eth0 inet static
    address 104.236.230.182/18
    gateway 104.236.192.1

# control-alias eth0
iface eth0 inet static
    address 10.17.0.7/16

auto eth1
iface eth1 inet static
    address 10.132.136.184/16

Output from ifconfig:

> ifconfig
eth0      Link encap:Ethernet  HWaddr 0e:91:7a:63:c6:e1
          inet addr:104.236.230.182  Bcast:104.236.255.255  Mask:255.255.192.0
          inet6 addr: fe80::c91:7aff:fe63:c6e1/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:1352 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1096 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:582069 (582.0 KB)  TX bytes:307106 (307.1 KB)

eth1      Link encap:Ethernet  HWaddr 12:91:f5:bb:25:1a
          inet addr:10.132.136.184  Bcast:10.132.255.255  Mask:255.255.0.0
          inet6 addr: fe80::1091:f5ff:febb:251a/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:4 errors:0 dropped:0 overruns:0 frame:0
          TX packets:7 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:280 (280.0 B)  TX bytes:578 (578.0 B)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:160 errors:0 dropped:0 overruns:0 frame:0
          TX packets:160 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1
          RX bytes:11840 (11.8 KB)  TX bytes:11840 (11.8 KB)

and ip route:

> ip route
default via 104.236.192.1 dev eth0 onlink
10.132.0.0/16 dev eth1  proto kernel  scope link  src 10.132.136.184
104.236.192.0/18 dev eth0  proto kernel  scope link  src 104.236.230.182

makes everything look like it's configured, but I cannot use ifup or ifdown either.

I've searched for the error "RTNETLINK answers: File exists" but nothing has really helped.