Purchase Orders - Approval Hierarchy

Purchasing > Setup > Organizations > Financial Options > Human Resources 
To Use Approval Hierarchies,the checkbox needs to be enabled.
On enabling, it shows that Position Hierarchy is used and then only then Positions are enabled in the assignment screen. 
To use Job based approval, disable the use approval hierarchy checkbox.

Note: Though you create Position in the Human Resources , if the above setup is not done ,then the Field is greyed out.