Having the same issue. Never seen this error before.

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.