No mode-of-transport categories determined from the document

What causes this issue?

Message DG 401 in the Bayer SDP system.
To execute dangerous goods checks, the system uses sales and
distribution data to determine the validity areas and mode of transport
categories that are relevant for dangerous goods.
No/not all mode of transport categories could be determined from the
document because
No route stages were specified, and
No shipping types are entered in the route header, or
No main leg shipping types are entered in the route header for the route
specified.

System Response

The dangerous goods check was terminated.
Data in the dangerous goods document table cannot be edited in the SD
document even if template material processing is activated.
When the SD document is saved the incompleteness status is set,
preventing the document being processed further (for example, creating a
delivery)

How to fix this error?

Enter the shipping types for the route or maintain the route stages.

Error message extract from SAP system. Copyright SAP SE.