then you may need to debug, or eventually open a call at SAP.
the place where this is checked is in program WSREO012 in FORM CHECK_PLANNED_ORDER
the table that is checked there is filled with this code:
select matnr plwrk
into (tab_plaf_marc-matnr, tab_plaf_marc-werks)
from plaf
where matnr = p_matnr
group by matnr plwrk
order by matnr plwrk.
hence you have to search table PLAF with material number and planning plant.
If there is nothing in this select and still SAP issues this error then it must be a bug