You can create a purchase order (SPO) for a Pending article in two ways.

1) By multi selecting Articles: 
When multi selecting articles the supplier must have a valid purchase price. 

2) By clicking on the green box with an arrow on it or enter on the line. 
In the Purchase screen that appears, enter on the quantity or week you would like to place the order and you will be redirected to a pop-up where you can select an open purchase order (SPO) or create a new one, and select a supplier without a valid purchase price.