Order entity has the following distinction: since one of the attributes is the Order.customer reference attribute, you should define a view including this attribute (standard _local view does not include reference attributes).

Go to the DATA MODEL tab on the navigation panel, select the Order entity and click the New → View button. View designer page will open. Enter order-with-customer as the view name, click on customer attribute and select _minimal view for the Customer entity on the panel on the right.

Click OK on the top panel.

After that, select the Order entity and click New → Generic UI screen.

Select order-with-customer in the View fields for both browser and editor templates and click Create.

order-edit.xml and order-browse.xml items will appear in the Web Module on the GENERIC UI tab of the navigation panel.