What causes this issue?

You tried to assign an output type to a transportation planning point /
an external transportation planning system. The output type is already
used for another system. An output type can, however, only be used for
an external transportation planning system.

System Response

The transaction can not be completed with this entry.

How to fix this error?

Use an output type that is not yet being used for an external
transportation planning system. This output type should be a copy of
output type 'TPS0'. Do not forget to enter this output type in the
output determination procedure for shipping output (e.g. see reference
procedure 'V10TPS').


&HINTS&
If you do not assign an output type to a transportation planning
system, then automatic transmission of delivery changes will not be
made to this system, even if the output determination procedure is
maintained.

Error message extract from SAP system. Copyright SAP SE.