Unable to Approve a PO

 Hi

While trying to approve a PO getting below error

Error:    Line #1 Item must be of type 'OUTSIDE OPERATION' for an outside operation line

Cause:        You entered an invalid item for the line type that you entered.  You must enter a outside-operation item if you enter an outside-operation line type.  

Action:        Enter an outside-operation item.

How can I resolve the same?

Thanks
Santhosh