Enter a valid in-transit plant or storage location

What causes this issue?

The following possibilities lead to this exception:
You have entered an in-transit plant, but no in-transit storage location
in the transport system that is relevant for stock projection This
in-transit plant is already assigned to another transport system that is
also relevant for stock projection.
You have entered an in-transit storage location for the in-transit plant
that is assigned to the transport system This storage location is
already assigned to another transport system that is relevant for stock
projection.

System Response

Depending on how the new in-transit plant and storage location has been
assigned to the transport system, the following changes have taken
place:
You can enter the same in-transit plant for more than one transport
system.
You can enter different storage locations for the same in-transit plant.
This plant is assigned to different transport systems. You can also not
enter any storage location.
The system behaves as follows:
You enter an in-transit storage location.
If you enter the same in-transit plant and the same in-transit storage
location for different transport systems, the system checks whether or
not the <ZK>TS not relevant for stock projection</> indicator in the
master data for all the transport systems is set. If not, the system
automatically sets this indicator.
You do not enter an in-transit storage location.
If you enter the same in-transit plant for different transport systems,
the system checks whether or not the <ZK>TS not relevant for stock
projection</> indicator in the master data for all the transport systems
is set. If not, the system automatically sets this indicator.

How to fix this error?

Enter a different in-transit storage location for the in-transit plant,
or set the <ZK>TS not relevant for stock projection</> indicator in the
master data of all the transport systems that use this in-transit plant
and storage location.

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.