I tried to add Matter device (Sonoff Mini-D) directly to Hubitat C-7.
It says added successfully but gets stuck at "Pairing Matter device, current stage: searching for device."
I am using latest version of Hubitat and the Hubitat android app.
Wifi 2.4GHz.
How can I debug what is missing/wrong?

I had the exact same issue with a Govee light strip. I tried multiple times commissioning using Hubitat. It never worked. I thought it was a bad light strip so I returned it and got another one. Still the same thing. It would commission right away via Home Assistant no problem. My work around was to commission it via Home Assistant then generate a code and using that code being it into Hubitat. (The old method.). Still doing that it took multiple times for it to work.
I have since commissioned multiple other light strips and bulbs with Hubitat with absolutely no issue. Works fine. This leads me to believe there are still some companies out there that don't have matter figured out exactly or regardless of the protocol doing it slightly different.
I hope you find a way to make it work it will help the rest of us learn.
Same result with 5GHz.
The device works if I add it in the official eWeLink app, with both 2.4GHz and 5GHz wifi.
I think it is not intended to pair a matter device directly to HE, so maybe try to follow these instructions, worked fine for me so far:
I don't have another Matter controller.
Docs say:
Hubitat Elevation is a Matter controller . This means you can pair devices that support Matter to your hub.
You can pair a matter device directly to Hubitat.
You don't need another one...
Some devices are just ornery and likely need a firmware update. I've had no probs with adding meross or tplink matter devices directly to hubitat.
@bcopeland Thoughts?
That's why I am used to pair all my devices with Nest Hub or my homePod, just to make sure they get the latest firmware update. Sometimes also the manufacturer's app is needed.
Never had a Matter device not in need of an update. 