V-LAN firewall rule

Hi,

i am brand new to the Sophos and i have one XGS3300. He is my DHCP server and the address range is from 10.10.2.1 to 10.10.16.250

I have one 3CX phone server with 10.10.128.2 address in a VLAN.

I setup the VLAN, but when i try to setup the Firewall rule, the error shows " you must provide a network IP address"

what i do wrong?

I know the 3CX has his own WAN ip address from my ISP.....

thanks for any information