What causes this issue?

When you maintained the route stages, you forgot to maintain one or
several transportation connection points that are needed. However,
before you can carry out the activity selected, you must maintain all
transportation connection points in the existing stages.
Depending on the stage type, the following maintenance rules exist:
Type 1 (shipment) : You must maintain the beginning and end
points.
(In single-column entry you only maintain the
beginning and end points for the last stage.
This means that at least two points are
required.)
Type 2 (load transfer): You must maintain the beginning point.
Type 3 (border) : You must maintain the beginning point.

System Response

The system terminates the activity you selected. You cannot branch to a
different entry screen nor can you save your data at this point.

How to fix this error?

The system places the cursor on the incorrect stage with incomplete
route definitions.
Either complete the route definition or delete the stages with
incomplete transportation connection points and repeat the activity.

Error message extract from SAP system. Copyright SAP SE.