"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?