The CLI --validate flag allows an optional "mode" argument to specify whether to have full, limited, or no validation. If mode is not specified, it defaults to no validation.
As of Daffodil 3.0.0, this appears to be broken and the mode parameter is required. If you do not specify a mode, you get the error:
We need to fix this to make it optional again, or if that isn't possible for some reason, make it so the documentation is changed to make it clear that mode is a required argument.