Requisition created in iProcurement

Hi All, Created a requisition in iProcuremetn for an item.

While choosing the purchase order  number required ,i got the follwoing error.

A purchase order number cannot be assigned because at least one requisition line is an internal sourced line.

2: with out chaoosng the purchase order required filed ,requisition created and apporved, but unable to choose this requisition in auto create form.

Is this meas i can create PO thorugh auto create funcionaly only for service line tyles??

Thanks &Regards

mondeddu