I can manually start/restart the service just fine after boot.
journal after boot
root@vpn01:~# journalctl -u wg-quick@wg0.service -n 20
-- Logs begin at Mon 2020-07-13 18:19:09 CDT, end at Mon 2020-07-13 18:19:24 CDT. --
Jul 13 18:19:09 vpn01 systemd[1]: Starting WireGuard via wg-quick(8) for wg0...
Jul 13 18:19:09 vpn01 wg-quick[493]: [#] ip link add wg0 type wireguard
Jul 13 18:19:09 vpn01 wg-quick[493]: [#] wg setconf wg0 /dev/fd/63
Jul 13 18:19:10 vpn01 wg-quick[493]: [#] ip -4 address add 10.9.0.1/24 dev wg0
Jul 13 18:19:10 vpn01 wg-quick[493]: [#] ip link set mtu 1420 up dev wg0
Jul 13 18:19:10 vpn01 wg-quick[493]: [#] sysctl net.ipv4.conf.wg0.forwarding=1 net.ipv4.conf.$(sed -n 3p /run/dietpi/.network
).forwarding=1
Jul 13 18:19:10 vpn01 wg-quick[493]: sed: can't read /run/dietpi/.network: No such file or directory
Jul 13 18:19:10 vpn01 wg-quick[493]: net.ipv4.conf.wg0.forwarding = 1
Jul 13 18:19:10 vpn01 wg-quick[493]: sysctl: separators should not be repeated: ..forwarding
Jul 13 18:19:10 vpn01 wg-quick[493]: sysctl: cannot stat /proc/sys/net/ipv4/conf//forwarding: No such file or directory
Jul 13 18:19:10 vpn01 wg-quick[493]: [#] ip link delete dev wg0
Jul 13 18:19:17 vpn01 systemd[1]: wg-quick@wg0.service: Main process exited, code=exited, status=255/
EXCEPTION
Jul 13 18:19:17 vpn01 systemd[1]: wg-quick@wg0.service: Failed with result 'exit-code'.
Jul 13 18:19:17 vpn01 systemd[1]: Failed to start WireGuard via wg-quick(8) for wg0.
Here’s the journal after I start the service manually after bootup
Jul 13 18:22:03 vpn01 systemd[1]: Starting WireGuard via wg-quick(8) for wg0...
Jul 13 18:22:03 vpn01 wg-quick[857]: [#] ip link add wg0 type wireguard
Jul 13 18:22:03 vpn01 wg-quick[857]: [#] wg setconf wg0 /dev/fd/63
Jul 13 18:22:03 vpn01 wg-quick[857]: [#] ip -4 address add 10.9.0.1/24 dev wg0
Jul 13 18:22:03 vpn01 wg-quick[857]: [#] ip link set mtu 1420 up dev wg0
Jul 13 18:22:03 vpn01 wg-quick[857]: [#] sysctl net.ipv4.conf.wg0.forwarding=1 net.ipv4.conf.$(sed -n 3p /run/dietpi/.network
).forwarding=1
Jul 13 18:22:03 vpn01 wg-quick[857]: net.ipv4.conf.wg0.forwarding = 1
Jul 13 18:22:03 vpn01 wg-quick[857]: net.ipv4.conf.eth0.forwarding = 1
Jul 13 18:22:03 vpn01 wg-quick[857]: [#] sysctl net.ipv6.conf.$(sed -n 3p /run/dietpi/.network).accept_ra=2
Jul 13 18:22:03 vpn01 wg-quick[857]: net.ipv6.conf.eth0.accept_ra = 2
Jul 13 18:22:03 vpn01 wg-quick[857]: [#] sysctl net.ipv6.conf.wg0.forwarding=1 net.ipv6.conf.$(sed -n 3p /run/dietpi/.network
).forwarding=1
Jul 13 18:22:03 vpn01 wg-quick[857]: net.ipv6.conf.wg0.forwarding = 1
Jul 13 18:22:03 vpn01 wg-quick[857]: net.ipv6.conf.eth0.forwarding = 1
Jul 13 18:22:03 vpn01 wg-quick[857]: [#] iptables -A FORWARD -i wg0 -j ACCEPT; iptables -t nat -A POSTROUTING -o $(sed -n 3p
/run/dietpi/.network) -j MASQUERADE
Jul 13 18:22:03 vpn01 wg-quick[857]: [#] ip6tables -A FORWARD -i wg0 -j ACCEPT; ip6tables -t nat -A POSTROUTING -o $(sed -n 3
p /run/dietpi/.network) -j MASQUERADE
Jul 13 18:22:03 vpn01 systemd[1]: Started WireGuard via wg-quick(8) for wg0.
I had not been able to update Dietpi since 7.6 due to Wireguard not working, and finding this finally helped me fix the issue and now I am on the latest version.