Oracle not asking for Ship Method Code

Hi GurusWe are running into a strange issue. Recently we planned to use Trip planning and so creating a trip and attaching multiple order lines to it through multiple deliveries.
We are not adding any ship method code into that trip.

When trying to perform ship confirm, Oracle is not letting user to enter any Ship Method code nor its defaulting any value. The field is greyed out. There is a need for user to select a Ship Method while doing ship confirm of Trip.

Any pointers ??
Thanks