"Unknown error." in LC when trying to test to connected ios device

[Mike Kerner]

Mac OS 11.5.2
LC 9.6.2
XCode 12.1
I choose a device from the Development->Test Target menu, then select
Development->Test
We get as far as the app getting signed, and then the error is thrown.
I'm pretty sure the certs and profiles are current, since I manually
removed them and then used XC to reinstall them.
Suggestions?

-- 
On the first day, God created the heavens and the Earth
On the second day, God created the oceans.
On the third day, God put the animals on hold for a few hours,
   and did a little diving.
And God said, "This is good."