Hello Julian,
The page for managing accounts in the Identity Manager web UI is generic, to accommodate all of the different connectors we have. However, not all connectors have the same features and functionality so not every button on that page works for every connector.
The Lock/Unlock feature is present in a few Identity Manager connectors but not all of them. OS400 is one of our connectors were you cannot manage lock/unlock through Identity Manager. So, the error you're seeing is expected behavior.
The management of locking and unlocking OS400 accounts must be done naively on your endpoint.
If you'd like to see Lock/Unlock added as a property to the OS400 connector, please open up an enhancement request.
Thank you,
Jennifer Johnston