I'm trying to configure Wireguard VPN. I've installed server on the Debian (Kali) VM. Host is Windows 8.1. My laptop is connected to the wireless network (Gateway 192.168.0.1; ip address of the laptop/host 192.168.0.11)
I use Vbox Host-only adapter; Paravirtualized network.
Here is my configs.

Server:
[Interface]
Address = 192.168.56.102/24
SaveConfig = true
PostUp = iptables -A FORWARD -i %i -j ACCEPT; iptables -A FORWARD -o %i -j ACCEPT; iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
PostDown = iptables -D FORWARD -i %i -j ACCEPT; iptables -D FORWARD -o %i -j ACCEPT; iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE
ListenPort = 51820
PrivateKey = CItUYtI8ARszRzWyvYKlpANqXB0N3unJGaRaD0F/YVk=

[Peer]
PublicKey = h177AFmuWQ2eG90JQI6NNwv4HAaEOhvkKlSQhZZhky8=
AllowedIPs = 192.168.0.16/32

Client on Windows host:

[Interface]
PrivateKey = UHq/rnUGhJGLAI2EobROMeRox9QAxB6T6TTmWQPa6EQ=
Address = 192.168.0.16/32
DNS = 1.1.1.1

[Peer]
PublicKey = XxeNBF96tVn7M7MkrtSppLqAxxbuC8Re/TDguXsv3CY=
AllowedIPs = 0.0.0.0/0
Endpoint = 192.168.56.102:51820
PersistentKeepalive = 30

On the guest:
#ifconfig

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.56.102 netmask 255.255.255.0 broadcast 192.168.56.255
inet6 fe80::a00:27ff:fe67:4749 prefixlen 64 scopeid 0x20<link>
ether 08:00:27:67:47:49 txqueuelen 1000 (Ethernet)
RX packets 105 bytes 18471 (18.0 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 33 bytes 5084 (4.9 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 177 bytes 39452 (38.5 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 177 bytes 39452 (38.5 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

wg0: flags=209<UP,POINTOPOINT,RUNNING,NOARP> mtu 1420
inet 192.168.56.102 netmask 255.255.255.0 destination 192.168.56.102
unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00 txqueuelen 1000 (UNSPEC)
RX packets 2 bytes 296 (296.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 105 dropped 26 overruns 0 carrier 0 collisions 0

# sudo nano /etc/sysctl.conf
net.ipv4.ip_forward = 1
# sudo sysctl -p

# wg show wg0

Interface: wg0
public key: *.*=
private key: (hidden)
listening port: 51820

peer: *.*=
endpoint: 192.168.56.1:60240
allowed ips: 192.168.0.16/32
transfer: 296 B received, 3.07 KiB sent# sudo nano /etc/sysctl.conf

ping -c 4 192.168.56.102
PING 192.168.56.102 (192.168.56.102) 56(84) bytes of data.
64 bytes from 192.168.56.102: icmp_seq=1 ttl=64 time=0.035 ms
64 bytes from 192.168.56.102: icmp_seq=2 ttl=64 time=0.037 ms
64 bytes from 192.168.56.102: icmp_seq=3 ttl=64 time=0.039 ms
64 bytes from 192.168.56.102: icmp_seq=4 ttl=64 time=0.037 ms
--- 192.168.56.102 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3049ms
rtt min/avg/max/mdev = 0.035/0.037/0.039/0.001 ms

ping -c 4 192.168.0.16
PING 192.168.0.16 (192.168.0.16) 56(84) bytes of data.

--- 192.168.0.16 ping statistics ---
4 packets transmitted, 0 received, 100% packet loss, time 3058ms

For the server address I always used 192.168.56.102. For the client address (and the Allowed IPs in server config file) I have tried different ip addresses. 192.168.0.16 the one is not occupied on the network. 192.168.0.1 is our Gateway. When I've tried to use the ip address of my local machine/host (192.168.0.11) I've got the error on the client: object already exist. That is followed by deactivation of the client.

What is may be a problem? I did not configure network on Kali itself. I did opened 51820 port in Windows FW, and hopefully made port forwarding on the router.

5.7.0-kali-amd64 (Debian 9.3.0-14)