I guess this is a permissions problem.
Try to grant the particular user %ALTER privileges on the table. You can
do that by using the SM-Portal. See [Home] -> [Security] -> [Users]
You might want to read more about Caché Security in the documentation:
http://docs.intersystems.com/cache20091/csp/docbook/DocBook.UI.Page.cls?KEY=GCAS_sql
Hth,
Thomas