Why does this message appear: the purchase order contains incomplete lines?

The notification "The purchase order contains incomplete lines" usually appears when there is
a product (or multiple) that is duplicated in the purchase order.

When the double line is removed, handling the purchase order should proceed without any issues.

In this case, it could be a truly unintended double line, but if that is not the case, it can be made into one line with the desired correct quantity.