External planned orders cannot be created manually

What causes this issue?

You tried to create an external planned order, but this type of planned
orders can only be created in the external planning system (SAP
Integrated Business Planning).

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.