Any tcode that starts with a Z is a custom written transaction only in your SAP system.
It is probably just a copy of VB21 with additional functionality added, and probably still runs VB21 in the background which is why you would need authorization. Are you able to run ZVB21 successfully manually in SAP without going through Winshuttle?
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Michael Shaw | Master Data Analyst
Southeastern Mills, Inc. | 706-291-6528
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