This article covers the error messages that start with "The operation couldn't be completed" and how to fix them.
OSSystemExtensionErrorDomain error 10
If you receive an error message "OSSystemExtensionErrorDomain error 10," while installing mmhmm, it's because you do not have admin permissions on your Mac computer.
To resolve this, contact your computer's systems administrator so they can install mmhmm desktop using admin permissions. You will be able to use mmhmm once this error is resolved.