"Unexpected error" while extending Time Capsule with Airport Express I picked up an Airport Express before they disappeared from the market, with the objective of using its audio-out to connect remotely to a HiFi.

My set-up is as follows:


Time Capsule connected to cable modem. Internet + Wifi + time machine working properly.
New, not yet hooked, Airport Express.


I gave up after many failed attempts using AirPort Utility (under High Sierra 10.13.6 on a MacBook Pro) to extend the network: "An unexpected error occurred. Try again."

Connecting the Airport Express using Ethernet to its LAN line, I created an independent WiFi network. Now I can AirPlay form iOS just fine.

I can also AirPlay from the MacBook but only after I switch from the Time Capsule's Wifi to the Airport Express's Wifi.

How can I extend the network? I've tried resetting the unit by disconnecting power, holding the reset button for 10 seconds, and then reconnecting power while the reset button is still pressed; as well as switching Wi-Fi TCP/IP to "Link-Local Only" under "Configure IPv6".

I vaguely have the sense that it all worked nicely until I hit the "update firmware" red button for the Airport Express. Is there a way to regress the firmware?
Working solution : Follow suggested setup steps on iphone instead of computer.

I encountered the exact same problem as you while trying to set up my airport express to extend an existing network. All attemps at doing so failed from my computer (MacBook Pro 16-inch 2019 with macOS Catalina 10.15.3). After going through all the setup windows and setting up the airport Express' password, it said "an unexpected error occured". 

After about 20 excruciating attempts and 3 different connection methods, I finally decided to try and set it up with my phone (Iphone 7 with iOS 13.3.1). In the wifi menu, I selected the airport express at the bottom of the page in a different list from usual wifi. I then followed exactly the same steps as those for a computer set up but on my iPhone and it worked!!