Open Button in Purchase Order Shipments Inactive

Hello All,
I hope you can help with an answer as to why I can't click on the Open button in the Purchase Order Shipments form.

The proces I am following is:

1. Enter invoice header
2. Enter invoice line(s)
3. Click on the Match button
4. Click on the View PO button
5. The Purchase Order Shipments form opens - the Open button is inactive

What could I have missed in config that means the button is inactive?

Thanks.