Is this a command you are running on an iSeries?
From your previous posts looks like you ask iSeries questions.
ADDMBX is not a command supplied with the Opererating System.
This is either a home gron command or part of a package you are running on your machine.
You can do a DSPCMD ADDMBX
This will tell you what library it is in. That may help identifying whether part of package or home grown,
It will also tell you what MODE(S) this command can run it. If only Interactive modes are identified, it will not run in a batch CL program.