What causes this issue?

You have attempted to use the transportation planning point &V1&.
However, this can be only be used for planning by an external planning
system (interface SD-TPS).

System Response

The entry was not accepted.

How to fix this error?

Enter another transportation planning point. Use the F4 help for
additional support.

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.