Hi
You could follow below steps to meet your requirement.
Port Management > VLAN membership > add new VLAN20 (router will create the correspond DHCP subnet in sequence automatically)
goto DHCP setup page and choose the VLAN20 then chang the subne...
You could just ignore that messages, it's mapping to IP & MAC binding option state not an error message. And you won't be able to shield it."Block MAC address on the list with wrong IP address / Block MAC address not on the list"