Error in bapi BAPI_PR_CREATE while uploading

Question

HI Gurus,
        I have program for uploading PR with BAPI. BAPI_PR_CREATE.
while uploading without account assignment category  it is working fine.
while enter account assignment category  v need to enter sales order no and item no. that is mandatory field for account assignment category..
if using program its showing error.
No instance of object type Purchase Requisition has been created. External reference: # 1
Enter sales order
Sd document XXXXXXX is not in the database or has been archived.
but  I tried to create manually it is working fine.
I tried to upload with bapi directly with out z program.
tht time also it showing error
No instance of object type Purchase Requisition has been created. External reference: # 1
Enter sales order
If any one give solution
it is very helpful for me.

Answer 1

Hi
Thnaks for the Reply,
in normal scenario it ll upload base on header.
if header change it ll generate automatically requisition no.  So in program v used AT END OF pr_type. 
so if pr_type is diff it will work.
in my case only one header it will not change. v hav only one pr type.
it should generate based on Item.
If item 10 it should generate new requisition No.
my flat file is ,
pr type -  material  - pur org - plant - storage loc  - qty -  traking no - ....  pr item
xxx          abc           pqr        1000       xyz             100         11111               10
xxx          abc           pqr        1000       xyz              200        11111                20        
xxx          abc           pqr        1000       xyz              300       11111                 30
xxx          abc           pqr        1000       xyz              400        11111                10
xxx          abc           pqr        1000       xyz              500        11111                20
here header is same.. if pr item 10 it should generate new requisition No, means it should commit before value 10.
so before commit v hav to give the logic if item 10 it should generate for before value.
hope u got my requirement.