Request &1, route &2, leg &3: No means of transport/carrier was found

What causes this issue?

Possible causes:
You have not defined a transportation service provider for the leg.
The following is true for the transportation service provider that is
defined for the leg:
You have defined the transportation service provider as not permitted.
The transportation service provider and the products in the request are
incompatible with regard to the transportation group.
The products in the request cannot be transported in the means of
transport that were found.

System Response

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

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.