Having the same issue. Never seen this error before.
GKBrown
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Just figured this out myself. You need to create a specific type of certificate ("Developer ID"):
https://developer.apple.com/help/account/create-certificates/create-developer-id-certificates/
Once you have done this, download the certificate and import it into Keychain Access. You'll see it as "Developer ID Application: name (id)". The value for --mac-signing-key-user-name should be name.
Did you ever find an answer to this? I'm running into the same issue.