It's recommended to take the second Deco close to the main Deco to configure. Or connect the 2nd unit to Ethernet port of main Deco via a cable and reconfigure. If still no success, you can try to restart the original modem and all the deco units. Then, try to reconfigure after restart.

problem was fixed, it caused due to netgear d6400 modem router that was changed to be only modem, after that I changed it to other modem all configuration was work perfectly. thank you