If you see A1084 or A1088, the version of AirPort Utility in El Capitan does not support this older version of the AirPort Express. If you see A1264, then you might be able to use AirPort Utility on your El Capitan Mac if the firmware on the AirPort Express has been updated to the 7.6.1 level. What I meant was that, according to that document, you don't need AirPort Utility 5.6.1 to setup and manage your TimeCapsule, but you can use the latest AirPort Utility, at the moment v.6.3.6, as part of the El Capitan OS. What I can suggest is try resetting to factory default TimeCapsule and start over with AirPort Utility. Download AirPort Utility 5.6.1 for Windows AirPort Utility 5.6.1 allows you to configure and manage the new AirPort Express with simultaneous dual band 802.11n. For the latest information on AirPort software, check Software Update in System Preferences or the following Apple website: Apple Support Downloads. Download Apple AirPort Utility for Mac to setup and manage airport base stations. If you’re running High Sierra (10.13), Sierra (10.12), or El Capitan (10.11), upgrade to macOS Catalina from the App Store. If you’re running Lion (10.7) or Mountain Lion (10.8), you will need to upgrade to El Capitan (10.11) first. Learn more about how to upgrade to macOS Catalina.

Download Airport Utility For Mac El Capitan 8

Thanks to Cucho Olivares that commented my previous post, here's an update.
It is an update note that comes from Corey J. Mahler, the guy who wrote the AirPort Utility 5.6.1 Launcher.
Here's the note
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Download Airport Utility For Mac El Capitan Free

It would appear that Apple has changed something in the network stack in a recent update to Yosemite. This update has resulted in AirPort Utility v5.6.1 not being able to find some AirPort Units some of the time. While this is annoying, there is a simple fix: Connect the AirPort Unit you wish to view in AirPort Utility v5.6.1 to the Mac running the utility via a wired (i.e., Ethernet) connection. I have yet to hear of any case where this has not solved the connectivity issue.'

Download Airport Utility For Mac El Capitan Download

This is the link, the note is taken from

Download Airport Utility For Mac El Capitan Mac

AirPort Utility v5.6.1