I tried to add Meross MS600 to my C8. I configured it on Alexa and then try to add to HE using the matter id from Alexa,
Pairing failed at Send NOC and with this message
Pairing failed @ FindOperational with code 50 (Timeout)
I tried to reboot my Hubitat, reset the sensor, reinstall it on Alexa, still no luck.
I also had a similar problem the first time I tried to add my MS600 mmWave sensor to HE.
Honestly, I don't remember how did I succeed on my second attemp... Most probably I have deleted it from the first ecosystem it was added (Apple Home), then it worked on the second try.