I read that "BEFORE triggers cannot modify the database"

Does this mean that Delete Statement shall not work with Before Triggers ?

Example:

Create Trigger delete_ROW_t1 NO cascade Before update of full_cat on CUST
referencing old as oldrow New as Newrow for each row mode db2sql
when (Newrow.full_cat = 1)
Begin Atomic
delete from C_PROD where id_cust = newrow.id_cust
End;

I am not able to execute the above statement. Is this because DELETE is not allowed with Before Trigger?

Please explain.

Thanks,

yes .. why not this delete in the after trigger ?

Yes, you can't use DELETE with a BEFORE trigger.
If you try, the create trigger will fail with sqlcode = -797 / -20100

Only an AFTER trigger can modify data.