Ribbon Workbench is not installed correctly
You can safely delete the Ribbon Workbench solution and then re-install it. When installing - ensure that you leave 'Enable any SDK processing steps' CHECKED!
If the issue still persists then create an un-managed solution and add the 'Sdk Message Processing Step' named 'Ribbon Workbench Action'. Check the status columns - if it says 'Disabled' then click 'Activate'.