To insert a line number in a purchase order, there must be a line number available.
At the item number input field, press F2 to go back to the line number input field and put in the line number to insert.

Another option is to remove line items that you want to insert.

As you are entering line items on a purchase order, you might want to consider using (M)essage lines between line items so you can go back and insert lines later.

Once you re-display the line item detail, the inserted line items will display
in their proper location.