System &1 has transport routes, but no standard layer for it

What causes this issue?

The system &V1& is the starting point for the consolidation routes. No
transportable transport layer or default transport layer has been
assigned to this system or to a client of this system. New packages and
Customizing settings are recorded in local requests.

System Response

This is a warning.

How to fix this error?

Check and if necessary, correct the default transport layer(s) for the
system &V1&.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.