Missing "Create Purchase Order Line" button

Paste http://folio-testing.aws.indexdata.com/orders into the browser's URL slot and press enter.
Click on 268758 to open that purchase order.
Click the "Add PO Line" button.
The "Add PO Line" form opens.
It has a red message "! Required fields!".
Enter 1 into the "PO Line Number" input field.
Enter 1 into the Barcode input field.
The red message "! Required fields!" disappears.

Expected: There is a "Create Purchase Order Line" button (or some indication about missing or wrong input in some input field).
Actual: No "Create Purchase Order Line" button, now way to create a purchase order line.

Setting to P2 because it blocks ui-erm.