Creation of Inter company Subcontracting PO

Hi experts,

I have 2 company codes .

1710 company code --> has plant 1710

1720 Company Code--> has plant 1720.

I need to create a Inter company subcontracting PO. I have BP supplier by name BP1710 for which a plant 1710 has been assigned.

Now I am trying a subcontracting PO in plant 1720 and vendor BP1710. The system considers its as STO process and throws up a error message as below

"no delivery type defined for supplying plant 1710 and document type NB "

Please suggest what I am doing wrong.