Ubuntu server 20.04: set ip isc-dhcp-server: network failed

I have set up isc-dhcp-server over Ubuntu server 20.04. Isc-dhcp-server starts well.

To set up my local network, I used:

• a pc to host isc-dhcp-server
• a pc as a client
• a switch to interconnect server and client

Below is the isc-dhcp-server status:

sudo systemctl restart isc-dhcp-server sudo systemctl status isc-dhcp-server.service

isc-dhcp-server.service - ISC DHCP IPv4 server
   Loaded: loaded (/lib/systemd/system/isc-dhcp-server.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2021-06-10 21:44:03 GMT; 3s ago
     Docs: man:dhcpd(8)
 Main PID: 6526 (dhcpd)
    Tasks: 1 (limit: 4915)
   CGroup: /system.slice/isc-dhcp-server.service
           └─6526 dhcpd -user dhcpd -group dhcpd -f -4 -pf /run/dhcp-server/dhcpd.pid -cf /etc/dhcp/dhcpd.conf br-6bc01dce843b

juin 10 21:44:03 tt-TECRA-R700 dhcpd[6526]: PID file: /run/dhcp-server/dhcpd.pid
juin 10 21:44:03 tt-TECRA-R700 dhcpd[6526]: Wrote 0 leases to leases file.
juin 10 21:44:03 tt-TECRA-R700 sh[6526]: Wrote 0 leases to leases file.
juin 10 21:44:04 tt-TECRA-R700 dhcpd[6526]: Listening on LPF/br-6bc01dce843b/02:42:f0:1e:4f:21/172.18.0.0/16
juin 10 21:44:04 tt-TECRA-R700 sh[6526]: Listening on LPF/br-6bc01dce843b/02:42:f0:1e:4f:21/172.18.0.0/16
juin 10 21:44:04 tt-TECRA-R700 sh[6526]: Sending on   LPF/br-6bc01dce843b/02:42:f0:1e:4f:21/172.18.0.0/16
juin 10 21:44:04 tt-TECRA-R700 sh[6526]: Sending on   Socket/fallback/fallback-net
juin 10 21:44:04 tt-TECRA-R700 dhcpd[6526]: Sending on   LPF/br-6bc01dce843b/02:42:f0:1e:4f:21/172.18.0.0/16
juin 10 21:44:04 tt-TECRA-R700 dhcpd[6526]: Sending on   Socket/fallback/fallback-net
juin 10 21:44:04 tt-TECRA-R700 dhcpd[6526]: Server starting service.

But when I plug a network cable to link the server to the switch, I get the following message:

network failed - Connection activation failed

I tried to connect a client to the switch so that it could get an address from the dhcp server, but client doesn't take any address at all.

$ ifconfig

br-6bc01dce843b: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet 172.18.0.1  netmask 255.255.0.0  broadcast 172.18.255.255
        ether 02:42:f0:1e:4f:21  txqueuelen 0  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

docker0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet 172.17.0.1  netmask 255.255.0.0  broadcast 172.17.255.255
        ether 02:42:0e:7d:a0:2a  txqueuelen 0  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

enp0s25: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        ether 00:23:18:41:b2:fb  txqueuelen 1000  (Ethernet)
        RX packets 1538  bytes 272584 (272.5 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 8126  bytes 1449096 (1.4 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device interrupt 20  memory 0xd4700000-d4720000  

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  (Boucle locale)
        RX packets 465692  bytes 44635267 (44.6 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 465692  bytes 44635267 (44.6 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

wlp2s0b1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.101  netmask 255.255.255.0  broadcast 192.168.1.255
        inet6 fe80::de66:bfea:9ee9:d266  prefixlen 64  scopeid 0x20<link>
        ether 4c:ed:de:54:ae:7d  txqueuelen 1000  (Ethernet)
        RX packets 3915334  bytes 3388537159 (3.3 GB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 3286057  bytes 606308993 (606.3 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

$ sudo gedit /etc/default/isc-dhcp-server

#INTERFACESv4="wlp2s0b1"
INTERFACESv4="br-6bc01dce843b"
INTERFACESv6=""

sudo gedit /etc/dhcp/dhcpd.conf

# dhcpd.conf

option domain-name "example.org";
option domain-name-servers ns1.example.org, ns2.example.org;

default-lease-time 600;
max-lease-time 7200;

ddns-update-style none;

authoritative;

subnet 172.18.0.0 netmask 255.255.0.0 
{
   range 172.18.0.10 172.18.0.50;
#  option routers 172.18.0.1;
   option subnet-mask 255.255.0.0;
#  option broadcast-address 172.18.255.255;
}