Trying to set up airport extreme an unexpected error occurs

I have this exact problem as well. In my case, I'm trying to configure a second Airport Extreme (AC) as an extender for my other Airport Extreme (AC).

I did the factory reset as per the instructions on Apple Support. When I first selected the second unit, I was asked to rename it if I wanted, so I did. I clicked Continue (all of this in Airport Utility) and got the message that said "An Unexpected Error Occured. Try Again."

Each time I try this, I get the same error message.

Am I doing something wrong?

Attached are a couple of screenshots of what I'm seeing:

I tried the settings in the article you linked to, but the problem remains.

I wonder if the problem is that I'm not doing a full factory reset on the Airport Extreme I'm trying to use as an extender? If you've read the instructions on how to do this, you almost need three hands, as you're supposed to hold the reset button in with a pen/paperclip, and plug the unit into the wall while observing the lights on the router. Pretty difficult to do, from what I'm seeing.

If your using a mac computer try your iphone instead. (Setting>WiFi>Join Network: Airport Extreme. After the initial setup is done on your iphone go to your mac to tweek your settings.

Hi,

You may find an answer in this thread. An unexpected error

The following article is a bit old but the advice to reset IPv6 might be worth trying.

AirPort base station not seen or "An unexpected error occurred" appears in AirPort Utility 6.3 - Apple Support