Please, could you share the error you are getting?
Try doing it Source Pre SQL or Source Post SQL. It should help.
Please share db version, product version & the error.
Having said that; please note at the session level, if DB2 target table properties have "truncate table" option set, PowerCenter tries up to four different approaches to perform the truncate in the following order:
truncate table <schema>.<table>;
import from /dev/null replace into <schema>.<table>;
CALL QSYS.QCMDEXC(' clrpfm file(<schema>/<table>)', <length>)
delete from <schema>.<table>;
If any of them succeeds, the remaining approaches are not executed and PowerCenter proceeds as if the table has been successfully truncated.
Note that "truncate table <schema>.<table>" requires that the DB2 user has the "ALTER TABLE" privilege on the table being truncated.